There are two times this tends to come up:

1. Have nuked the motor/ESC through yet again exceeding the recommended amp draw by 75% (or motor has failed in some other way)
or
2. Have run the battery dead.

Excluding my trainer, I've probably flown around 850 flights. I've probably lost power on less than 10 occasions:

Motor slipped it's mount - 1
Motor failed after collision with another aircraft - 1
Ran out of batteries - 4
RX/TX Failure - 3

7 of those problems were on one aircraft. By and large epower is very reliable. Just use a kitchen timer or something to track your airtime, and don't let flying buddies goad you into racing them when you know your battery is low (ask me how I know).