Once made an intentionaal gear up landing with a 76, well nose wheel up that is. Arrived home base and put gear down only to get the mains locked and green. Few recycles with no joy, a fly by of the tower showed mains down, nose wheel retracted with doors closed. Landed with the nose sitting on a pile of sand bags. Cause was the oleo didn't extend on the previous take off with the result the scissors jammed up against the hydraulic jack and bent the actuating rod protuding from the piston.