Once had a helicopter student, just prior to his final handling test get cognitive failure during an engine off landing. Instead of raising the collective at about ten feet AGL, to arrest the descent and cushion the touchdown he pulled the cyclic hard back. I was already close by on the controls and rapidly took control but not quite soon enough to prevent the tail just touching the grass, resulting in minor damage. He’d flown a near perfect trip to that point, it was the last item on the sortie profile.
Taught me that any student, no matter how good he flies, can throw a no notice wobbler.