Why one of the teams in the DEF CON social engineering contest failed. The team's representative was trying to get a woman on the phone to give him some information by telling her that his brother, who was sitting right there in front of him, was deaf, and wouldn't she please help out his brother? He could give her his cell phone number.

I can't remember the details. Sorry.

Some time passes, and he pleads with her a little more, until she finally says "Why does your deaf brother need a cell phone?"

a moment of silence
...and hundreds of people bust out laughing.