Stupid mobs resetting all the time!
For example: skurv farmers in ishalgen/altgard/morheim, meek crestlich in beluslan. When you start hitting them they run away, when they are done running they go back to idle/patrol and they have full hp again, sometimes this happens 5 times in a row on 1 mob!
Besides that, there are also weird resets with mobs that are dotted, mobs that reset while being attacked by Ice Sheet, mobs that reset after/while being cc'ed or feared... etc.