When the stupid question trick didn't work ...

When it is just 5 minutes left and the professor is about to start a new topic, somebody should make a sacrifice, ruin his reputation and do the stupid question trick because otherwise you have to sit there for another 15 minutes before the professor realizes that he has gone 10 minutes overtime.
How does it work? ... Simple, just make a complete fool of yourself and ask a very stupid question. The professor will be busy answering your question for the remaining time of the class.
But when the trick doesn't work, it really hurts ...