Didnt hear this one my self, but I have been told about it.....
A lufftansa A340 was blocking traffic on the ramp in boston one day, and the controller said for the lufftansa plane to move.
The lufftansa pilots responded: We cant we are missing 3 passengers.
A pilot from one of the other planes said: Did you check your ovens!

Than it went dead silent, no body said anything.