Cessna 152.
It used to yaw badly in flight, various attempts to cure it failed, eventually fault made itself apparent. It was a late 152 with the landing lights in the wing and the Perspex window had worked itself loose, at certain speeds the window would vibrate across opening the leading edge up and causing the aircraft to yaw, on landing it would do the reverse so all appeared normal, it was only when it decided to stick open was the fault finally identified and fixed.