Many years ago, there was a rogue C130 based out in the Far East, every so often in the cruise it would pitch up for no reason. No fault could be found after extensive work then some bright spark noticed that every time it happened, the same loadie was on board. It turned out he was disaffected and thought it a good wheeze to swing on the control runs every now and then.
Re the Varsity incident detailed earlier. According to mates who were there at the time, it was standard practice to try to barrel roll the aircraft when on a student mutual GH ride. On this occasion, the handling pilot (CR I believe) became unhappy half way round and pulled through rather than continue the roll.