There has been at least two cases recently of the main rotor blades as a whole detaching in flight. Gearbox failure caused the second crash [1]
On the older 'teetering head' designs like the Huey, you can bump the mast (rotor shaft) in aggressive nose overs or low g conditions which would also separate the rotors from the aircraft.
[0] https://aviation-safety.net/wikibase/wiki.php?id=213384
[1] https://en.wikipedia.org/wiki/CHC_Helikopter_Service_Flight_...
