diff --git a/CHANGELOG.md b/CHANGELOG.md
index 110b512..94fe7bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 
 **Improvements**
 
+* Add `!` syntax to expect command failures (panics or errors).
 * Add `Command.consume_args()` for convenient argument handling.
 * Allow `@` in unquoted strings.
 
diff --git a/src/command.rs b/src/command.rs
index bdbdd64..cdf2b35 100644
--- a/src/command.rs
+++ b/src/command.rs
@@ -26,6 +26,9 @@ pub struct Command {
     /// Silences the output of this command. This is handled automatically, the
     /// [`Runner`](crate::Runner) does not have to take this into account.
     pub silent: bool,
+    /// If true, the command is expected to fail with a panic or error. If the
+    /// command does not fail, the test fails.
+    pub fail: bool,
     /// The command's line number position in the script.
     pub line_number: u32,
 }
diff --git a/src/lib.rs b/src/lib.rs
index 1ea10a5..b665a16 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -187,9 +187,9 @@
 //!     ```
 //!
 //! * [**Prefix:**](Command::prefix) an optional :-terminated string prefix
-//!   before the command name. The command's output will be given the same
-//!   prefix. The prefix can be used by the test runner, e.g. to signify two
-//!   different clients.
+//!   before the command. The command's output will be given the same prefix.
+//!   The prefix can be used by the test runner, e.g. to signify two different
+//!   clients.
 //!
 //!     ```text
 //!     client1: put key=value
@@ -210,6 +210,21 @@
 //!     foo
 //!     ```
 //!
+//! * [**Failure:**](Command::fail) if `!` precedes the command, it is expected
+//!   to fail with an error or panic, and the failure message is used as output.
+//!   If the command unexpectedly succeeds, the test fails. If the line contains
+//!   other symbols before the command name (e.g. a prefix or silencing), the
+//!   `!` must be used immediately before the command name.
+//!
+//!     ```text
+//!     ! command error=foo
+//!     prefix: ! command panic=bar
+//!     (!command error=foo)
+//!     ---
+//!     Error: foo
+//!     prefix: Panic: bar
+//!     ```
+//!
 //! ## Output
 //!
 //! The command output following a `---` separator can contain any arbitrary
diff --git a/src/parser.rs b/src/parser.rs
index 586a1e7..d9c69b3 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -98,8 +98,12 @@ fn command(input: Span) -> IResult<Command> {
     let (input, maybe_silent) = opt(terminated(char('('), space0))(input)?;
     let silent = maybe_silent.is_some();
 
-    // The command itself.
+    // The prefix and fail marker.
     let (input, prefix) = opt(terminated(identifier, pair(tag(":"), space0)))(input)?;
+    let (input, maybe_fail) = opt(terminated(char('!'), space0))(input)?;
+    let fail = maybe_fail.is_some();
+
+    // The command itself.
     let line_number = input.location_line();
     let (input, name) = identifier(input)?;
     let (mut input, args) = many0(preceded(space1, argument))(input)?;
@@ -114,7 +118,7 @@ fn command(input: Span) -> IResult<Command> {
     let (input, _) = opt(comment)(input)?;
     let (input, _) = line_ending(input)?;
 
-    Ok((input, Command { name, args, prefix, silent, line_number }))
+    Ok((input, Command { name, args, prefix, silent, fail, line_number }))
 }
 
 /// Parses a single command argument, consisting of an argument value and
diff --git a/src/runner.rs b/src/runner.rs
index 2e65267..250de5c 100644
--- a/src/runner.rs
+++ b/src/runner.rs
@@ -116,16 +116,52 @@ pub fn generate<R: Runner>(runner: &mut R, input: &str) -> std::io::Result<Strin
         ));
 
         for command in &block.commands {
-            // Execute the command.
-            let mut command_output = runner.run(command).map_err(|e| {
-                std::io::Error::new(
-                    std::io::ErrorKind::Other,
-                    format!(
-                        "command '{}' failed at line {}: {e}",
-                        command.name, command.line_number
-                    ),
-                )
-            })?;
+            // Execute the command. Handle panics and errors if requested. We
+            // assume the command is unwind-safe when handling panics, it is up
+            // to callers to manage this appropriately.
+            let run = std::panic::AssertUnwindSafe(|| runner.run(command));
+            let mut command_output = match std::panic::catch_unwind(run) {
+                // Unexpected success, error out.
+                Ok(Ok(output)) if command.fail => {
+                    return Err(std::io::Error::new(
+                        std::io::ErrorKind::Other,
+                        format!(
+                            "expected command '{}' to fail at line {}, succeeded with: {output}",
+                            command.name, command.line_number
+                        ),
+                    ))
+                }
+
+                // Expected success, output the result.
+                Ok(Ok(output)) => output,
+
+                // Expected error, output it.
+                Ok(Err(e)) if command.fail => format!("Error: {e}"),
+
+                // Unexpected error, return it.
+                Ok(Err(e)) => {
+                    return Err(std::io::Error::new(
+                        std::io::ErrorKind::Other,
+                        format!(
+                            "command '{}' failed at line {}: {e}",
+                            command.name, command.line_number
+                        ),
+                    ))
+                }
+
+                // Expected panic, output it.
+                Err(panic) if command.fail => {
+                    let message = panic
+                        .downcast_ref::<&str>()
+                        .map(|s| s.to_string())
+                        .or_else(|| panic.downcast_ref::<String>().cloned())
+                        .unwrap_or_else(|| std::panic::resume_unwind(panic));
+                    format!("Panic: {message}")
+                }
+
+                // Unexpected panic, throw it.
+                Err(panic) => std::panic::resume_unwind(panic),
+            };
 
             // Silence the output if requested.
             if command.silent {
diff --git a/tests/errors/fail_before_prefix.error b/tests/errors/fail_before_prefix.error
new file mode 100644
index 0000000..2a8c695
--- /dev/null
+++ b/tests/errors/fail_before_prefix.error
@@ -0,0 +1,3 @@
+parse error at line 1 column 8 for CrLf:
+!prefix: command
+       ^
\ No newline at end of file
diff --git a/tests/errors/fail_before_prefix.in b/tests/errors/fail_before_prefix.in
new file mode 100644
index 0000000..dd00047
--- /dev/null
+++ b/tests/errors/fail_before_prefix.in
@@ -0,0 +1,2 @@
+!prefix: command
+---
\ No newline at end of file
diff --git a/tests/errors/fail_before_silence.error b/tests/errors/fail_before_silence.error
new file mode 100644
index 0000000..edf2a30
--- /dev/null
+++ b/tests/errors/fail_before_silence.error
@@ -0,0 +1,3 @@
+parse error at line 1 column 2 for Tag:
+!(command)
+ ^
\ No newline at end of file
diff --git a/tests/errors/fail_before_silence.in b/tests/errors/fail_before_silence.in
new file mode 100644
index 0000000..b404a16
--- /dev/null
+++ b/tests/errors/fail_before_silence.in
@@ -0,0 +1,2 @@
+!(command)
+---
\ No newline at end of file
diff --git a/tests/errors/fail_error.error b/tests/errors/fail_error.error
new file mode 100644
index 0000000..0f3db04
--- /dev/null
+++ b/tests/errors/fail_error.error
@@ -0,0 +1 @@
+command '_error' failed at line 1: message
\ No newline at end of file
diff --git a/tests/errors/fail_error.in b/tests/errors/fail_error.in
new file mode 100644
index 0000000..711aafa
--- /dev/null
+++ b/tests/errors/fail_error.in
@@ -0,0 +1,2 @@
+_error message
+---
\ No newline at end of file
diff --git a/tests/errors/fail_ok_unexpected.error b/tests/errors/fail_ok_unexpected.error
new file mode 100644
index 0000000..ddf6233
--- /dev/null
+++ b/tests/errors/fail_ok_unexpected.error
@@ -0,0 +1 @@
+expected command '_echo' to fail at line 1, succeeded with: message
\ No newline at end of file
diff --git a/tests/errors/fail_ok_unexpected.in b/tests/errors/fail_ok_unexpected.in
new file mode 100644
index 0000000..695abec
--- /dev/null
+++ b/tests/errors/fail_ok_unexpected.in
@@ -0,0 +1,2 @@
+! _echo message
+---
\ No newline at end of file
diff --git a/tests/errors/fail_panic.error b/tests/errors/fail_panic.error
new file mode 100644
index 0000000..36ebb02
--- /dev/null
+++ b/tests/errors/fail_panic.error
@@ -0,0 +1 @@
+message
\ No newline at end of file
diff --git a/tests/errors/fail_panic.in b/tests/errors/fail_panic.in
new file mode 100644
index 0000000..0f2f0cf
--- /dev/null
+++ b/tests/errors/fail_panic.in
@@ -0,0 +1,2 @@
+_panic message
+---
\ No newline at end of file
diff --git a/tests/generate/empty_output.out b/tests/generate/empty_output.out
index f8f9c85..1fdc180 100644
--- a/tests/generate/empty_output.out
+++ b/tests/generate/empty_output.out
@@ -2,12 +2,12 @@
 # an empty block at the end of the file.
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 3 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 3 }
 
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 6 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 6 }
 
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 9 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 9 }
diff --git a/tests/scripts/commands b/tests/scripts/commands
index 9d7ef68..7df3a4f 100644
--- a/tests/scripts/commands
+++ b/tests/scripts/commands
@@ -1,35 +1,40 @@
 # A bare command.
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 2 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 2 }
 
 # Commands with arguments.
 foo arg
 bar key=value
 baz arg key=value
 ---
-Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, line_number: 7 }
-Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, line_number: 8 }
-Command { name: "baz", args: [Argument { key: None, value: "arg" }, Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, line_number: 9 }
+Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: false, line_number: 7 }
+Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, fail: false, line_number: 8 }
+Command { name: "baz", args: [Argument { key: None, value: "arg" }, Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, fail: false, line_number: 9 }
 
 # Commands with prefixes.
 a: foo arg
 b: bar key=value
 ---
-a: Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: Some("a"), silent: false, line_number: 16 }
-b: Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: Some("b"), silent: false, line_number: 17 }
+a: Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: Some("a"), silent: false, fail: false, line_number: 16 }
+b: Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: Some("b"), silent: false, fail: false, line_number: 17 }
+
+# Failing commands.
+! foo bar
+---
+Error: Command { name: "foo", args: [Argument { key: None, value: "bar" }], prefix: None, silent: false, fail: true, line_number: 23 }
 
 # Both prefixes, commands, and argument identifiers can be whitespace, but not
 # empty (tested under errors/).
 " ": " " " "=" "
 ---
- : Command { name: " ", args: [Argument { key: Some(" "), value: " " }], prefix: Some(" "), silent: false, line_number: 24 }
+ : Command { name: " ", args: [Argument { key: Some(" "), value: " " }], prefix: Some(" "), silent: false, fail: false, line_number: 29 }
 
 # Empty argument values are fine.
 command ""
 command arg=""
 command arg=
 ---
-Command { name: "command", args: [Argument { key: None, value: "" }], prefix: None, silent: false, line_number: 29 }
-Command { name: "command", args: [Argument { key: Some("arg"), value: "" }], prefix: None, silent: false, line_number: 30 }
-Command { name: "command", args: [Argument { key: Some("arg"), value: "" }], prefix: None, silent: false, line_number: 31 }
+Command { name: "command", args: [Argument { key: None, value: "" }], prefix: None, silent: false, fail: false, line_number: 34 }
+Command { name: "command", args: [Argument { key: Some("arg"), value: "" }], prefix: None, silent: false, fail: false, line_number: 35 }
+Command { name: "command", args: [Argument { key: Some("arg"), value: "" }], prefix: None, silent: false, fail: false, line_number: 36 }
diff --git a/tests/scripts/comments b/tests/scripts/comments
index 5828deb..302911c 100644
--- a/tests/scripts/comments
+++ b/tests/scripts/comments
@@ -5,25 +5,25 @@
 # Comment before command.
 command id=1
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, line_number: 6 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, fail: false, line_number: 6 }
 
 command id=2 # Comment beside command.
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, line_number: 10 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, fail: false, line_number: 10 }
 
 command id=3
 # Comment after command.
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "3" }], prefix: None, silent: false, line_number: 14 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "3" }], prefix: None, silent: false, fail: false, line_number: 14 }
 
 # Comment between blocks.
 
 command id=4
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "4" }], prefix: None, silent: false, line_number: 21 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "4" }], prefix: None, silent: false, fail: false, line_number: 21 }
 
 command id=5 // Comment using //.
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "5" }], prefix: None, silent: false, line_number: 25 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "5" }], prefix: None, silent: false, fail: false, line_number: 25 }
 
 # Comment at end.
\ No newline at end of file
diff --git a/tests/scripts/dos_line_endings b/tests/scripts/dos_line_endings
index e293b59..a9cb7fb 100644
--- a/tests/scripts/dos_line_endings
+++ b/tests/scripts/dos_line_endings
@@ -2,7 +2,7 @@
 
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 3 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 3 }
 
 // Comment.
 
@@ -10,9 +10,9 @@ foo arg
 bar key=value
 baz arg key=value
 ---
-Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, line_number: 9 }
-Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, line_number: 10 }
-Command { name: "baz", args: [Argument { key: None, value: "arg" }, Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, line_number: 11 }
+Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: false, line_number: 9 }
+Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, fail: false, line_number: 10 }
+Command { name: "baz", args: [Argument { key: None, value: "arg" }, Argument { key: Some("key"), value: "value" }], prefix: None, silent: false, fail: false, line_number: 11 }
 
 # Comment.
 a: foo arg
@@ -20,7 +20,7 @@ a: foo arg
 b: bar key=value
 # Comment.
 ---
-a: Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: Some("a"), silent: false, line_number: 18 }
-b: Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: Some("b"), silent: false, line_number: 20 }
+a: Command { name: "foo", args: [Argument { key: None, value: "arg" }], prefix: Some("a"), silent: false, fail: false, line_number: 18 }
+b: Command { name: "bar", args: [Argument { key: Some("key"), value: "value" }], prefix: Some("b"), silent: false, fail: false, line_number: 20 }
 
 # Comment.
\ No newline at end of file
diff --git a/tests/scripts/fail b/tests/scripts/fail
new file mode 100644
index 0000000..fe8809b
--- /dev/null
+++ b/tests/scripts/fail
@@ -0,0 +1,27 @@
+# ! is parsed correctly into the fail attribute. It is output as an error, since
+# the runner expects an error.
+! command arg
+---
+Error: Command { name: "command", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: true, line_number: 3 }
+
+# Errors and panics are handled when ! is given.
+! _error foo
+---
+Error: foo
+
+! _panic foo
+---
+Panic: foo
+
+# () can be used to silence the output.
+(!_error foo)
+(!_panic foo)
+---
+ok
+
+# Prefixes can be used too.
+a: ! _error foo
+b:!_panic foo
+---
+a: Error: foo
+b: Panic: foo
diff --git a/tests/scripts/hooks b/tests/scripts/hooks
index e3a2dce..8445c6c 100644
--- a/tests/scripts/hooks
+++ b/tests/scripts/hooks
@@ -2,13 +2,13 @@
 _set start_block="start" end_block="end"
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 3 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 3 }
 end
 
 command
 ---
 start
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 8 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 8 }
 end
 
 # Newlines in block hooks should be handled appropriately.
@@ -16,13 +16,13 @@ _set start_block="start\n" end_block="end\n"
 command
 ---
 start
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 16 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 16 }
 end
 
 command
 ---
 start
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 22 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 22 }
 end
 
 # As should empty lines.
@@ -30,7 +30,7 @@ _set start_block="start\n\n" end_block="end\n\n"
 command
 ---
 > start
-> Command { name: "command", args: [], prefix: None, silent: false, line_number: 30 }
+> Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 30 }
 > end
 > 
 
@@ -39,5 +39,5 @@ command
 ---
 > start
 > 
-> Command { name: "command", args: [], prefix: None, silent: false, line_number: 38 }
+> Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 38 }
 > end
diff --git a/tests/scripts/output b/tests/scripts/output
index 94bd1e8..8397f1b 100644
--- a/tests/scripts/output
+++ b/tests/scripts/output
@@ -2,8 +2,8 @@
 command
 command arg
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 2 }
-Command { name: "command", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, line_number: 3 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 2 }
+Command { name: "command", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: false, line_number: 3 }
 
 # Output that contains empty lines should automatically be prefixed with >.
 # This should be the case for empty lines at the start and end of the output,
@@ -13,17 +13,17 @@ command id=1
 command id=2
 ---
 > 
-> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, line_number: 12 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, fail: false, line_number: 12 }
 > 
-> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, line_number: 13 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, fail: false, line_number: 13 }
 
 _set prefix="" suffix="\n\n"
 command id=1
 command id=2
 ---
-> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, line_number: 21 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, fail: false, line_number: 21 }
 > 
-> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, line_number: 22 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, fail: false, line_number: 22 }
 > 
 
 _set prefix="\n" suffix="\n\n"
@@ -31,10 +31,10 @@ command id=1
 command id=2
 ---
 > 
-> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, line_number: 30 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, fail: false, line_number: 30 }
 > 
 > 
-> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, line_number: 31 }
+> Command { name: "command", args: [Argument { key: Some("id"), value: "2" }], prefix: None, silent: false, fail: false, line_number: 31 }
 > 
 
 # Empty output blocks should default to "ok", but only once, and only if none of
@@ -54,7 +54,7 @@ ok
 command
 (command)
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 54 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 54 }
 
 _set start_block="" end_block="end"
 ---
diff --git a/tests/scripts/silent b/tests/scripts/silent
index 8254bdc..5146c8f 100644
--- a/tests/scripts/silent
+++ b/tests/scripts/silent
@@ -8,8 +8,8 @@ command id=1
 (command id=2)
 command id=3
 ---
-Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, line_number: 7 }
-Command { name: "command", args: [Argument { key: Some("id"), value: "3" }], prefix: None, silent: false, line_number: 9 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "1" }], prefix: None, silent: false, fail: false, line_number: 7 }
+Command { name: "command", args: [Argument { key: Some("id"), value: "3" }], prefix: None, silent: false, fail: false, line_number: 9 }
 
 # Whitespace is allowed around the parentheses, except for the first.
 ( command  )  # eol
diff --git a/tests/scripts/strings b/tests/scripts/strings
index 7a75c73..a8816b5 100644
--- a/tests/scripts/strings
+++ b/tests/scripts/strings
@@ -2,18 +2,18 @@
 0: 1 2 3=4
 01: 23 45 67=89
 ---
-0: Command { name: "1", args: [Argument { key: None, value: "2" }, Argument { key: Some("3"), value: "4" }], prefix: Some("0"), silent: false, line_number: 2 }
-01: Command { name: "23", args: [Argument { key: None, value: "45" }, Argument { key: Some("67"), value: "89" }], prefix: Some("01"), silent: false, line_number: 3 }
+0: Command { name: "1", args: [Argument { key: None, value: "2" }, Argument { key: Some("3"), value: "4" }], prefix: Some("0"), silent: false, fail: false, line_number: 2 }
+01: Command { name: "23", args: [Argument { key: None, value: "45" }, Argument { key: Some("67"), value: "89" }], prefix: Some("01"), silent: false, fail: false, line_number: 3 }
 
 # Unquoted strings can start with _.
 _prefix: _command _arg _key=_value
 ---
-_prefix: Command { name: "_command", args: [Argument { key: None, value: "_arg" }, Argument { key: Some("_key"), value: "_value" }], prefix: Some("_prefix"), silent: false, line_number: 9 }
+_prefix: Command { name: "_command", args: [Argument { key: None, value: "_arg" }, Argument { key: Some("_key"), value: "_value" }], prefix: Some("_prefix"), silent: false, fail: false, line_number: 9 }
 
 # Unquoted strings can contain -_./@
 prefix-_.: command-_./@ arg-_./@ key-_./@=value-_./@
 ---
-prefix-_.: Command { name: "command-_./@", args: [Argument { key: None, value: "arg-_./@" }, Argument { key: Some("key-_./@"), value: "value-_./@" }], prefix: Some("prefix-_."), silent: false, line_number: 14 }
+prefix-_.: Command { name: "command-_./@", args: [Argument { key: None, value: "arg-_./@" }, Argument { key: Some("key-_./@"), value: "value-_./@" }], prefix: Some("prefix-_."), silent: false, fail: false, line_number: 14 }
 
 # Single-quoted strings can contain any character, including newlines.
 '➡️': '😀' '"👋"' '\t'='\0' '
@@ -22,7 +22,7 @@ prefix-_.: Command { name: "command-_./@", args: [Argument { key: None, value: "
 
 '
 ---
-➡️: Command { name: "😀", args: [Argument { key: None, value: "\"👋\"" }, Argument { key: Some("\t"), value: "\0" }, Argument { key: None, value: "\n\n  🚀\n\n" }], prefix: Some("➡\u{fe0f}"), silent: false, line_number: 19 }
+➡️: Command { name: "😀", args: [Argument { key: None, value: "\"👋\"" }, Argument { key: Some("\t"), value: "\0" }, Argument { key: None, value: "\n\n  🚀\n\n" }], prefix: Some("➡\u{fe0f}"), silent: false, fail: false, line_number: 19 }
 
 # Double-quoted strings can too.
 "➡️": "😀" "'👋'" "\t"="\0" "
@@ -31,37 +31,37 @@ prefix-_.: Command { name: "command-_./@", args: [Argument { key: None, value: "
 
 "
 ---
-➡️: Command { name: "😀", args: [Argument { key: None, value: "'👋'" }, Argument { key: Some("\t"), value: "\0" }, Argument { key: None, value: "\n\n  🚀\n\n" }], prefix: Some("➡\u{fe0f}"), silent: false, line_number: 28 }
+➡️: Command { name: "😀", args: [Argument { key: None, value: "'👋'" }, Argument { key: Some("\t"), value: "\0" }, Argument { key: None, value: "\n\n  🚀\n\n" }], prefix: Some("➡\u{fe0f}"), silent: false, fail: false, line_number: 28 }
 
 # Single- and double-quoted strings can also be empty, but is not allowed in
 # identifiers (prefixes, commands, and argument names). It is allowed as
 # argument values.
 command foo="" bar=''
 ---
-Command { name: "command", args: [Argument { key: Some("foo"), value: "" }, Argument { key: Some("bar"), value: "" }], prefix: None, silent: false, line_number: 39 }
+Command { name: "command", args: [Argument { key: Some("foo"), value: "" }, Argument { key: Some("bar"), value: "" }], prefix: None, silent: false, fail: false, line_number: 39 }
 
 # Single- and double-quoted strings respect all of the same escape sequences,
 # including both quote types.
 '\\ \' \" \0 \n \r \t \\'
 "\\ \' \" \0 \n \r \t \\"
 ---
-Command { name: "\\ ' \" \0 \n \r \t \\", args: [], prefix: None, silent: false, line_number: 45 }
-Command { name: "\\ ' \" \0 \n \r \t \\", args: [], prefix: None, silent: false, line_number: 46 }
+Command { name: "\\ ' \" \0 \n \r \t \\", args: [], prefix: None, silent: false, fail: false, line_number: 45 }
+Command { name: "\\ ' \" \0 \n \r \t \\", args: [], prefix: None, silent: false, fail: false, line_number: 46 }
 
 # Quoted strings can contain the other, unescaped quote kind.
 '"'
 "'"
 ---
-Command { name: "\"", args: [], prefix: None, silent: false, line_number: 52 }
-Command { name: "'", args: [], prefix: None, silent: false, line_number: 53 }
+Command { name: "\"", args: [], prefix: None, silent: false, fail: false, line_number: 52 }
+Command { name: "'", args: [], prefix: None, silent: false, fail: false, line_number: 53 }
 
 # Quoted strings can also contain special characters like silencing ( and prefix
 # : without them being interpreted as such.
 '(command:' arg
 ---
-Command { name: "(command:", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, line_number: 60 }
+Command { name: "(command:", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: false, line_number: 60 }
 
 # They can also contain comments.
 'command # with comment'
 ---
-Command { name: "command # with comment", args: [], prefix: None, silent: false, line_number: 65 }
+Command { name: "command # with comment", args: [], prefix: None, silent: false, fail: false, line_number: 65 }
diff --git a/tests/scripts/whitespace b/tests/scripts/whitespace
index 8094008..1bdcd07 100644
--- a/tests/scripts/whitespace
+++ b/tests/scripts/whitespace
@@ -8,7 +8,7 @@
 
 command
 ---
-Command { name: "command", args: [], prefix: None, silent: false, line_number: 9 }
+Command { name: "command", args: [], prefix: None, silent: false, fail: false, line_number: 9 }
 
 
   # Comment.
@@ -18,7 +18,7 @@ Command { name: "command", args: [], prefix: None, silent: false, line_number: 9
 
 command     arg
 ---
-Command { name: "command", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, line_number: 19 }
+Command { name: "command", args: [Argument { key: None, value: "arg" }], prefix: None, silent: false, fail: false, line_number: 19 }
 
 
 
diff --git a/tests/tests.rs b/tests/tests.rs
index 4267e4c..bfa8a37 100644
--- a/tests/tests.rs
+++ b/tests/tests.rs
@@ -27,25 +27,36 @@ fn test_generate([in_path, out_path]: [&std::path::Path; 2]) {
 }
 
 // Generate error tests for each pair of *.in and *.error files in tests/errors/.
-// The input scripts are expected to error with the stored output.
+// The input scripts are expected to error or panic with the stored output.
 test_each_path! { for ["in", "error"] in "tests/errors" as errors => test_error }
 
 fn test_error([in_path, out_path]: [&std::path::Path; 2]) {
     let input = std::fs::read_to_string(in_path).expect("failed to read file");
-    let error =
-        goldenscript::generate(&mut DebugRunner::new(), &input).expect_err("script succeeded");
+    let run =
+        std::panic::AssertUnwindSafe(|| goldenscript::generate(&mut DebugRunner::new(), &input));
+    let message = match std::panic::catch_unwind(run) {
+        Ok(Ok(_)) => panic!("script succeeded"),
+        Ok(Err(e)) => e.to_string(),
+        Err(panic) => panic
+            .downcast_ref::<&str>()
+            .map(|s| s.to_string())
+            .or_else(|| panic.downcast_ref::<String>().cloned())
+            .unwrap_or_else(|| std::panic::resume_unwind(panic)),
+    };
 
     let dir = out_path.parent().expect("invalid path");
     let filename = out_path.file_name().expect("invalid path");
     let mut mint = goldenfile::Mint::new(dir);
     let mut f = mint.new_goldenfile(filename).expect("failed to create goldenfile");
-    f.write_all(error.to_string().as_bytes()).expect("failed to write goldenfile");
+    f.write_all(message.as_bytes()).expect("failed to write goldenfile");
 }
 
 /// A goldenscript runner that debug-prints the parsed command. It
 /// understands the following special commands:
 ///
 /// _echo: prints back the arguments, one per line
+/// _error: errors with the given string
+/// _panic: panics with the given string
 /// _set: sets various options
 ///
 ///   - prefix=<string>: printed immediately before the command output
@@ -53,6 +64,8 @@ fn test_error([in_path, out_path]: [&std::path::Path; 2]) {
 ///   - start_block=<string>: printed at the start of a block
 ///   - end_block=<string>: printed at the end of a block
 ///
+/// If a command is expected to fail via !, the parsed command string is
+/// returned as an error.
 #[derive(Default)]
 struct DebugRunner {
     prefix: String,
@@ -80,6 +93,16 @@ impl goldenscript::Runner for DebugRunner {
                 command.args.iter().map(|a| a.value.clone()).collect::<Vec<String>>().join(" ")
             }
 
+            "_error" => {
+                let message = command.args.first().map(|a| a.value.as_str()).unwrap_or("error");
+                return Err(message.to_string().into());
+            }
+
+            "_panic" => {
+                let message = command.args.first().map(|a| a.value.as_str()).unwrap_or("panic");
+                panic!("{message}");
+            }
+
             "_set" => {
                 for arg in &command.args {
                     match arg.key.as_deref() {
@@ -94,6 +117,8 @@ impl goldenscript::Runner for DebugRunner {
                 return Ok(String::new());
             }
 
+            _ if command.fail => return Err(format!("{command:?}").into()),
+
             _ => format!("{command:?}"),
         };