This is a true story told to me by a friend and edited down a bit.

Kevin, on holiday with some friends, walks into a bar and sees his friends already there at a table, except for one, Larry, who was standing at the far end of the bar waiting for his order of drinks. There's a sign posted on the wall, visible just above Larry's head. It says "No Gambling".

On coming inside Kevin spots Larry and then the sign. He points at it, calling out in a voice loud enough to reach Larry, and therefore everyone else, "I bet they don't enforce that here!"

That slowly gathers the bar's patron's attentions as Larry swivels his head up to read the sign. The bartender is just finishing Larry's drinks and is walking back to him as Larry looks back from the sign and replies in a booming voice, "You're on!"

Both guys have big smiles on their faces as everyone in the place turns to look at the bartender to see which one wins the bet. A few seconds goes by with all eyes on the hapless barman, caught in a logical trap, and then the whole place erupts into laughter.