Man in a cinema watching a cowboy film. A stagecoach pulls up, man 1 turns to his neighbour and says "I bet you a tenner that the first cowboy bangs his head on the doorframe." Man 2 accepts the bet. The first guy out banged his head so man 2 pays up.
Man 1 feels guilty and gives back the tenner, says "I'm sorry, I saw this film last week, it made me feel bad taking your money."
Man 2 says "So did I, but I didn't think he'd be daft enough to do it again."