If it happens when your enemy executes a power attack on you I have a bad news for you. It's either a bug or intended design decision. If that power attack have enough power to one shot you, the control over your character gets removed and you can't avoid that incoming hit. A video to illustrated it:
http://www.youtube.com/watch?v=arPRsagOJ8k
I have created a topic about it here:
http://www.gamesas.com/index.php?/topic/1284157-a-major-bug-in-power-attack-code/page__gopid__19386290#entry19386290
Feel free to contribute.