Ive known of 3-4 occurences. All as detailed above.
Heavy landings by students, during engine off landing training (of course, one could argue the instructors intervention was too late but thats a small glasshouse to throw stones in!!).
Only once did the blades actually sever the tailboom itself. On all other occasions the blades flexed enough to strike the aft short shaft of the tail rotor drive. As the aircraft were already on the ground there was no loss of directional control.