An engineer was scheduled to be executed during the French Revolution. He was strapped into the guillotine, and the blade was raised, up, and up, and up, and at the top the executioner pulled the string. Down fell the blade, stopping six inches from the exposed neck of the victim.
"Sacre bleu! Let him up," said the executioner. The engineer got up, examined the track, and exclaimed "oh, here - I've found the problem..."