The AI's movements in Global Agenda, if they don't know where you are/you're too far away.
Basically, if they don't know where to run to, they occasionally enter a random movement pattern phase, where they stop/start left/right back/forwards for a while. This can make me look like a crappy sniper on occasion - I don't mind people, at least they move for a reason!
Any AI that cheats.
Those damn 'floor masters' in Zelda: The Windwaker, they were such a pain, untill I worked out that overcautiousness was key.
When mobs in RPGs decide they won't attack you any more, and regain full health while running back to their starting position.
When the tower in League of Legends/DotA (mainly DotA) decide not to attack the enemy hero, and focus the minions instead, and conversely, when they focus you when they should be focusing minions.
SO many more, but hey, listing them all would be effort.