Irish Bank Robber
kvalidir's excellent Irish sausages joke reminds me of the failed Irish bank robber. He had a very broad accent and every time he went into a bank and shouted "Stick 'em up" the teller would double up laughing and say "You're Irish, aren't you?".
Determined to overcome this he worked hard on his accent until it had disappeared completely.
The next time he was on a job he walked into the bank and said to the teller "I say, you chap, hands up", at which point the teller doubled up laughing and said "You're Irish, aren't you?". "How do you know?" asked the robber, "I haven't got an accent". "Well", replied the teller, "It's your saw-off shotgun. You've sawn the wrong end off".