Driving force, I've heard a zipper story too, but it was a little different.
A guy goes up to give his talk and his zipper was down. He notices, and leans over to zip it, and his tie gets caught in his zipper. He tries to undo it but it will NOT budge, and he can't stand up straight. A helpful brother comes running down the aisle with a pair of scissors, and snips his tie in two. The guy stands up, and the lower half of his tie flops comically down.