diff --git a/src/core/derivation.ts b/src/core/derivation.ts
index 203b65425..7599b8ccc 100644
--- a/src/core/derivation.ts
+++ b/src/core/derivation.ts
@@ -135,8 +135,17 @@ export function isComputingDerivation() {
 }
 
 export function checkIfStateModificationsAreAllowed(atom: IAtom) {
+    const hasObservers = atom.observers.size > 0
+    // Should never be possible to change an observed observable from inside computed, see #798
+    if (globalState.computationDepth > 0 && hasObservers)
+        fail(
+            process.env.NODE_ENV !== "production" &&
+                `Computed values are not allowed to cause side effects by changing observables that are already being observed. Tried to modify: ${
+                    atom.name
+                }`
+        )
     // Should not be possible to change observed state outside strict mode, except during initialization, see #563
-    if (!globalState.allowStateChanges && (atom.observers.size > 0 || globalState.enforceActions === "strict"))
+    if (!globalState.allowStateChanges && (hasObservers || globalState.enforceActions === "strict"))
         fail(
             process.env.NODE_ENV !== "production" &&
                 (globalState.enforceActions
diff --git a/test/base/action.js b/test/base/action.js
index b8db892ec..0e96f07a5 100644
--- a/test/base/action.js
+++ b/test/base/action.js
@@ -202,30 +202,6 @@ test("should be possible to change observed state in an action called from compu
     d()
 })
 
-test("should be possible to change observed state in an action called from computed if run inside _allowStateChangesInsideComputed - 2", () => {
-    const a = mobx.observable.box(2)
-    const d = mobx.autorun(() => {
-        a.get()
-    })
-
-    const testAction = mobx.action(() => {
-        a.set(3)
-    })
-
-    const c = mobx.computed(() => {
-        mobx._allowStateChanges(true, () => {
-            testAction()
-        })
-        return a.get()
-    })
-
-    c.get()
-
-    mobx._resetGlobalState()
-    d()
-})
-
-
 test("should not be possible to change observed state in an action called from computed", () => {
     var a = mobx.observable.box(2)
     var d = mobx.autorun(() => {