What I hate the most is when projectile-based enemies attack in directions they're not facing. For instance, last night I was playing Castlevania: Harmony of Despair (I'm 2 years late to the party, I know

) and was playing as Charlotte trying to farm Dracula's Hellfire spell. I found the best way to avoid is Dark Inferno and Flame Pillar attacks are to simply jump over him and stand behind him. Well that's all fine and good until you notice him lifting the arm that signals Pillar or Inferno, jump behind him, and have the inferno balls fire out backwards through his cape to nail you in the face and kill you. >.>
And lets talk about the AI-thrown grenades in Mass Effect 3. I can't tell you how many times I'd be in a flanking position on some Cerberus soldier, attacking him from the side, for instance, only to have him stand and chuck a grenade - while facing my squadmates - perpendicularly at a 90 degree angle from the direction he's facing to have it land right at my feet. The worst I've seen of this, the guy straight up threw it behind him while facing and "throwing" forward.
Then there's the grenade indicator on your Hud...apparently the up and down arrows don't mean "in front of you" or "behind you" as you'd expect them to. No, apparently the up arrow means the grenade is still in the air, and the down arrow means it's officially on the ground. So you see a grenade get thrown and all of a sudden there's a red flashing arrow pointing down. CoD instincts would have you say "OH SHIT!" and try to roll or dodge forward, away from the grenade. Well in ME 3, 9 times out of 10, you just rolled right on top of the grenade and are now officially kerploded.
And of course, there's the times when they pull off psychic trick-shots with their grenades. Apparently knowing that your instinct will be to rush their position and do a heavy melee when they throw the grenade at you, they'll intentionally throw it too high so that it bounces off something on the ceiling and falls right by their feet. You arrive to deliver the melee and BOOM! Shepard gets his legs blown off. >.>
Edit: Oh, and apparently some Cerberus soldiers have the Martyrdom perk, as they'll drop/throw grenades despite being caught in a Pull or Singularity.