Here is a pull request for 1.12.5 as well. #3995 As for proper fix in the current state of the code I have an idea. Instead of returning false or blocking we can do the following. Add a new method in NotificationQueue similar to dequeueNotification but instead of returning a task which needs to be executed now that it returns the next task in queue regardless of when it needs to be executed. Let's call that method dequeueNextNotification, I am not stuck on naming it's just for reference now. Execution of Cancel notification would turn to something like this then. size_t pendingSize = queue().size(); while (pendingSize > 0) { Poco::AutoPtr<TimerNotification> pNf = static_cast<TimerNotification*>(queue().dequeueNextNotification()); pendingSize--; ... } So we would get the size of pending tasks when we start to execute Cancel. We want to dequeue that many notifications regardless of when they need to be executed. We flush one by one taking special consideration of stop and cancel tasks as before. After we leave the while loop we will again remove the clear() method. Does that make sense? I can also write all of that down in the issue itself, and possibly take care of that in the coming weeks. Vojin Ilic