Akin posted a record in Skill Bug Reports
[Lord Knight] Berserk/Frenzy - HP/SP Draining Effect 2.0
This is basically the same bug as this report. It was fixed but for some reason it occurs again. I tested it on iRO last time and confirmed the behaviour is not normal.
Skill Name: Berserk/Frenzy iRO
Job Name: Rune knight