The one I heard about was the instructor that use to take his student in an aircraft which had wooden sticks that could be removed to aid getting in and out. The stick slid into place in a metal square beneath the seat.
Well the instructor was well known for getting the novice student to do a T and G on the field or a low level pass, at which point, with great flurry he would throw his stick out of the aircraft. He would tell the student he had no alternative but to land the aircraft himself next time around, of course knowing full well that he simply had to ask the student to slide out his stick and pass it over if the need arose.
Well one wag knew the procedure well and so also threw his stick out immediately after the instructor proclaiming that he assumed the instructor was demonstrating what he wanted him to do.
You can imagine the instructors ashen face realising that they didnt have a stick between them and no way of controlling the aircraft.
Fortunately the student had secreted another stick by their side.
It was the last time the instructor tried that one.