A man walks into a bar, drinks a couple of beers, and prepares to leave. The bartender tells him he owes $8. "But I already paid you. Don't you remember? Says the customer. "OK," says the bartender, "If you say you paid, then I suppose you did."
The man goes outside and tells the first person he sees that the bartender can't keep track of whether his customers have paid or not. The second man rushes in, orders a couple beers, and later pulls the same stunt. The barkeep replies, "OK, If you say you paid, than I suppose you did."
The customer goes outside and tells a friend how to get free drinks. The third man hurries into the bar and begins to drink highballs. The bartender leans over and says, "You know, a funny thing happened tonight. Two men were drinking beer, neither paid, and both claimed they had. The next guy who tries that stunt is going to get punched in the face"
The man interrupts, "Don't bother me with your troubles, bartender. Just give me my bill and ill be on my way."