diff --git a/changelog.d/19960.bugfix b/changelog.d/19960.bugfix
new file mode 100644
index 00000000000..c2af8dd7838
--- /dev/null
+++ b/changelog.d/19960.bugfix
@@ -0,0 +1 @@
+Fix a bug where `/sync` could return a 500 error ("Missing state for event that is not user's own membership") when an out-of-band membership event, since de-outliered into the room's current state, appeared in a room timeline.
diff --git a/synapse/visibility.py b/synapse/visibility.py
index 4eeba14a6c5..5b8e180f9ac 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -196,21 +196,17 @@ def allowed(event: EventBase) -> FilteredEvent | None:
         # Filter out call invites in public rooms, as this would potentially
         # ring a lot of users.
         if event.type == EventTypes.CallInvite and not event.is_state():
-            # `state_after_event` should only be None if the event is an outlier,
-            # and earlier code should filter out outliers entirely.
-            #
-            # In addition, we only create outliers locally for out-of-band
-            # invite rejections, invites received over federation, or state
-            # events needed to authorise other events. None of this applies to
-            # call invites.
-            assert state_after_event is not None
-
-            room_join_rules = state_after_event.get((EventTypes.JoinRules, ""))
-            if (
-                room_join_rules is not None
-                and room_join_rules.content.get("join_rule") == JoinRules.PUBLIC
-            ):
-                return None
+            # `state_after_event` should only be None if the event is an outlier
+            # (or our copy of the event is stale and still flagged as one, see
+            # below); in either case we can't check the join rules, so let the
+            # event through.
+            if state_after_event is not None:
+                room_join_rules = state_after_event.get((EventTypes.JoinRules, ""))
+                if (
+                    room_join_rules is not None
+                    and room_join_rules.content.get("join_rule") == JoinRules.PUBLIC
+                ):
+                    return None
 
         # Annotate the event with the user's membership after the event.
         #
@@ -224,8 +220,23 @@ def allowed(event: EventBase) -> FilteredEvent | None:
         elif state_after_event is not None:
             user_membership_event = state_after_event.get((EventTypes.Member, user_id))
         else:
-            # unreachable!
-            raise Exception("Missing state for event that is not user's own membership")
+            # We have no state for this event: it was flagged as an outlier
+            # when we fetched the state above, but `always_include_ids` let it
+            # through anyway. This can happen when an out-of-band membership
+            # event (e.g. an invite, or a rejection of one, received while not
+            # in the room) is later de-outliered — because we are also in the
+            # room and received it over federation — and enters the current
+            # state, while the copy of the event we are working with (e.g. from
+            # a worker's not-yet-invalidated cache) still carries the outlier
+            # flag. We don't know the user's membership at this point in the
+            # DAG, which MSC4115 permits — return the event unannotated rather
+            # than failing the whole request.
+            logger.info(
+                "filter_and_transform_events_for_client: no state for always-included"
+                " outlier %s; returning it without a membership annotation",
+                event.event_id,
+            )
+            return FilteredEvent(event=filtered, membership=None)
 
         user_membership = (
             user_membership_event.membership
diff --git a/tests/test_visibility.py b/tests/test_visibility.py
index 9a5efbdd399..257e6feeda4 100644
--- a/tests/test_visibility.py
+++ b/tests/test_visibility.py
@@ -665,6 +665,31 @@ def test_out_of_band_invite_rejection(self) -> None:
             [],
         )
 
+        # ... unless the events are in `always_include_ids`. This mirrors what
+        # happens when an out-of-band membership event is later de-outliered
+        # (because we are also in the room and received it over federation) and
+        # enters the current state — which sync passes as `always_include_ids`
+        # — while the copy of the event being filtered still carries the
+        # outlier flag. We can't compute the requesting user's membership at
+        # such an event (we have no state for it), but it must be let through
+        # without a membership annotation rather than failing the whole
+        # request.
+        #
+        # Regression test for https://github.com/element-hq/synapse/issues/19858.
+        filtered_events = self.get_success(
+            filter_and_transform_events_for_client(
+                self.hs.get_storage_controllers(),
+                "@other:test",
+                [invite_event, reject_event],
+                always_include_ids=frozenset({invite_event_id}),
+            )
+        )
+        self.assertEqual(
+            [e.event.event_id for e in filtered_events],
+            [invite_event_id],
+        )
+        self.assertEqual([e.membership for e in filtered_events], [None])
+
 
 async def inject_visibility_event(
     hs: HomeServer,