PIN

He stood at the cashpoint, a gun against his back.
"Don't try anything funny."
He wouldn't dare. He slid his card into the slot and paused as the machine asked for his PIN.
He'd heard that if you were ever in this situation that you should enter your PIN in reverse and the police would be there within minutes. He wasn't sure if this was true, but he thought it was worth a try.
He entered his PIN, backwards, and withdrew £500 which he gave to the robber.
The criminal ran off with his haul, but the police were nowhere in sight.
The man hadn't realised that they would never come, whether there was truth in the PIN thing or not, as his number was a numerical palindrome.