I remember just before my UH-1B Squadron had a series of inflight rotor separations, that the Engineering Officer came into the crewroom and had us look down inside a supposedly serviceable mast that had been just removed from a machine I'd flown the day before. It appeared alarmingly ovulated via that internalised view (holding it up to the light) but you just couldn't tell it by feel or appearance on the external viewing. Within days we'd lost the first crew and there were to be two more before we stopped flying and waited on Ma Bell's boys to find the fix. Supposedly this was "no more touch-down auto's (with floored collective and mast-bumping on rollout)"
The hierarchy wouldn't go along with that but luckily we got some D models to play with. We were very ignorant. No one told me that if I used a downwind hover reference that the hot air pooling around the T/R drive-shaft hangar-bearing beneath the jet efflux would cause a lubrication breakdown and a tail-rotor drive-shaft failure in a 120 ft hover. But it was always easier to learn that way, because you never forgot.