A man is in a bar and has a couple of drinks. The bartender tells him that he owes $10.
"But, bartender," the man says, "I already paid you! Don't you remember?"
"Well, okay," replies the bartender, "if you say you've already paid, then I guess you have."
The man goes outside, approaches the first person he sees, and tells him that the bartender can't seem to keep track of whether his customers have paid or not. On hearing that, the second man rushes into the bar, orders a beer, and then pulls the same stunt on the bartender.
"Okay," says the bartender, "if you say you've already paid, then I guess you have."
The second man goes outside, sees a friend, and tells him how to get free drinks.
The third man rushes into the bar and starts gulping down one drink after the other. A short time later, the bartender leans over and says, "You know, a funny thing happened in here tonight. Two men were drinking, neither one paid, but both claimed that they had. The next guy that tries that stunt is going to get his butt kicked."
"Listen, bartender," the man interrupts, "don't bother me with your problems. Just give me my change and I'll be on my way."