I reproduced a couple times an assertion failure due to removing a not-added timeout in this spot, during dbus_shutdown(). I think the below would fix it. It's a little hard to be sure since it's so hard to reproduce in the first place. The backtrace kind of sucks too because gcc is inlining the heck out of dbus-connection.c

The assertion failure is this one in dbus-timeout.c:

 if (!_dbus_list_remove (&timeout_list->timeouts, timeout))
    _dbus_assert_not_reached ("Nonexistent timeout was removed");

The patch just adds "if is_timeout_added(pending)"

@@ -2188,9 +2199,14 @@ connection_timeout_and_complete_all_pending_calls_unlocked (DBusConnection *conn
        
       _dbus_pending_call_queue_timeout_error_unlocked (pending, 
                                                        connection);
-      _dbus_connection_remove_timeout_unlocked (connection,
-                                                _dbus_pending_call_get_timeout_unlocked (pending));
-      _dbus_pending_call_set_timeout_added_unlocked (pending, FALSE);       
+
+      if (_dbus_pending_call_is_timeout_added_unlocked (pending))
+        {
+          _dbus_connection_remove_timeout_unlocked (connection,
+                                                    _dbus_pending_call_get_timeout_unlocked (pending));
+          _dbus_pending_call_set_timeout_added_unlocked (pending, FALSE);
+        }
+