chore: Add comment and warning to unhappy path

src/api/client/membership/leave.rs

@@ -178,7 +178,19 @@ pub async fn leave_room(
 					.rooms
 					.state_cache
 					.left_state(user_id, room_id)
-					.await?
+					.await
+					.inspect_err(|err| {
+						// `left_state` may return an Err if the user _is_ in the room they're
+						// trying to leave, but the membership cache is incorrect and
+						// they're cached as being joined. In this situation
+						// we save a `None` to the `roomuserid_leftcount` table, which generates
+						// and sends a dummy leave to the client.
+						warn!(
+							"Trying to leave room not cached as leave, sending dummy leave \
+							 event to client"
+						);
+					})
+					.unwrap_or_default()
 			},
 		}
 	};