over time the knotch on the flap handle which locks it in place wears away. Then, the flaps may still engage but every now and then will slip, jolting back to a clean wing.
I've had this happen a few times in the Chipmunk over the decades. Usually, on that aeroplane, it's the detent that drops into the slot that shears (and no, I wasn't exceeding VFE - I've seen those teeny actuating cables!).