I hadn't actually encountered this issue until today when dungeon diving in Arx Corinium in Shadowfen. I was at least 10 levels lower than all the enemies, and even though I wore heavy armor, the Resto Staff was a must. I have a particular ability under the restoration staff skill tree I believe that when I do a successful heavy attack, it restores 15% of my magicka which works especially well with my magic build. Though throughout the entire dungeon, the game was not allowing me to do heavy attacks. Just short zaps from the staff that I don't even believe did any damage, at least from what I was seeing. Occasionally the staff would do a heavy attack, but keeping the R2 button held with the enemy still in my sight glowing red, it would continue to zap. Again, I've never encountered this before, so perhaps maybe the dungeon itself was bugged. In any case, this situation was causing me great grief as I was not able to regenerate magic fast enough to heal anyone all because my staff refused to do any heavy attacks.

Not too big of a game breaker, but being that I had no magic drams on me, you can see how this little ability was a necessity. If I'm leaving out any other essential detail, please let me know.

(PS, I did actually manage to get through the dungeon anyways, for those curious )