A man in a bar has a couple of beers and the bartender tells him he owes $6.50.
The barkeeper replies, "If you say you paid, I'll take your word for it."
the bartender leans over and says, "You know, a funny thing happened in here tonight. Two men were drinking beer, neither paid and both claimed that they did. The next guy who tries that is going to get punched right on the face."