diff --git a/changelog.d/19711.doc b/changelog.d/19711.doc
new file mode 100644
index 00000000000..d00ee6a908e
--- /dev/null
+++ b/changelog.d/19711.doc
@@ -0,0 +1 @@
+Add warning about known problems when configuring `use_frozen_dicts`.
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index 48f33d54270..e58a2dcf10b 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -194,7 +194,11 @@ user_agent_suffix: ' (I''m a teapot; Linux x86_64)'
 ---
 ### `use_frozen_dicts`
 
-*(boolean)* Determines whether we should freeze the internal dict object in `FrozenEvent`. Freezing prevents bugs where we accidentally share e.g. signature dicts. However, freezing a dict is expensive. Defaults to `false`.
+*(boolean)* Determines whether we should freeze the internal dict object in `FrozenEvent`. Freezing prevents bugs where we accidentally share e.g. signature dicts. However, freezing a dict is expensive.
+
+> ⚠️ **Warning** – This option is known to introduce a new class of [comparison bugs](https://github.com/element-hq/synapse/issues/18117) in Synapse.
+
+Defaults to `false`.
 
 Example configuration:
 ```yaml
diff --git a/schema/synapse-config.schema.yaml b/schema/synapse-config.schema.yaml
index 6acc5c37625..2056e8b9a2a 100644
--- a/schema/synapse-config.schema.yaml
+++ b/schema/synapse-config.schema.yaml
@@ -94,6 +94,10 @@ properties:
       Determines whether we should freeze the internal dict object in
       `FrozenEvent`. Freezing prevents bugs where we accidentally share e.g.
       signature dicts. However, freezing a dict is expensive.
+
+
+      > ⚠️ **Warning** – This option is known to introduce a new class of [comparison
+      bugs](https://github.com/element-hq/synapse/issues/18117) in Synapse.
     default: false
     examples:
       - true
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index f4a5624d1a2..9656e578bc5 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -62,6 +62,10 @@
 NOTE: This is overridden by the configuration by the Synapse worker apps, but
 for the sake of tests, it is set here because it cannot be configured on the
 homeserver object itself.
+
+FIXME: Because of how this option works (changing the underlying types), it causes
+subtle downstream bugs that makes type comparisons brittle, tracked by
+https://github.com/element-hq/synapse/issues/18117
 """
 
 T = TypeVar("T")