I guess this could be by design, but seems kind of odd that berserk can still fail (rather often too) and expend the fail mana when it's at 100% - basically there's no benefit to mastering it except slightly smaller chance of failing.

The vast majority of abilities are affected by other stuff in addition to skill percentage.

In the case of berserk, the chance for success is:
skill% + 25 - (hit point %)/2

So going berserk at full health with 100% skill has a 75% of working, but will always succeed at 50% health.

nt

NT