The barman says: "That's $2.50, mate." And the guy replies: "Don't you remember? I've already paid you. I'm not gonna pay you again." "No problem, then" the barman replies with a smile.

Later on, when leaving the bar, the guy runs into one of his best friends, and tells him what happened. And sure enough, his mate also gets away with it.

As the second guy was leaving the pub, he tells a third mate about the situation.

So the third guy walks in, orders a beer, and tries to get away without paying. The barman says: "Y'know, mate, I've just had two guys come in here and have a drink without paying a cent. The next guy who tries that is gonna get his face punched in." The third guy goes: "Yeah, must be awful. Now are you gonna give me my change or what?"