Loss of tail rotor control: You are not able to control the tail rotor pitch mechanism - it is a true emergency. This could be a stuck control (left pedal, right pedal, or centre) or it can be a total loss of thrust (broken drive shaft or gear box). It can occur on any helo, but is relatively rare. Part of the training on every helo.
No, I disagree. These emergencies are different and should be treated as such.
A tail rotor driveshaft is more straightforward to deal with; usually this requires the engines to be shut down for landing.
If you did that when the tail rotor has run away to high or full positive pitch, it could be a fatal mistake.