Zombies in L4D2 (and first game too, probably).
You can never tell when they'll hit you (really poor animations), and they can't seem to be able to run in a straight line. They act as some sort of a supernatural zero inertia magnet-held-on-surface things that can do a 90/180 turn without slowing down on each their step.
Really pisses me off. One moment you're thinking "I've got 400msec to escape this", bit no... the hand animations are not synced to hits and movement has no inertia (as opposed to your character).
Really fucking pissing me off.
EDIT: And why is everything so supernatural... almost a fairytale. Zombies, fine. But don't make them run with their guts pouring, for eF's sake. It's impossible, as a human body wouldn't be able to bend forward because muscles are ripped.