COD Black Ops: Combat training on hardened difficulty. Sniping is already hard enough in this game due to wonky hit detection, but when a bot walking opposite your direction half a map away decides to suddenly 180-no-recoil-snipe you with a pithy SMG through a bush, it's time to exit the game before damaging something. Mind you, this happens all the time and I suspect the bots are biased against players. Everytime an enemy meets you and several of your teammates, and even if you're at the back of the crowd, you always miraculously die first. It's either badly-implemented bots or teams stacked against me 4/5 servers I join. *shrug* Sometimes I really want to like the game, but it doesn't seem to want to like me.
Serious Sam TFE: The frog room. Good God, the whole street must have heard my rage induced by the sheer unfairness of that section.
Crysis and the military choppers with spidey-sense. Even if you were cloaked and behind cover, they'd just hover around your position. Try and stealth away perfectly, and it STILL follows you around until one of you destroys the other.
Bad Company 2 and the rubber knife you wield at times, whilst some others are blessed with the almighty MW2 six-metre commando lunge.
STALKER series (on master difficulty): You can have a shiny and new, extremely accurate rifle but the instant you put a silencer on it, it shoots wildly. It's not helped by the fact that the AI can wield the worst pistol in the game that is in bad condition, or even sawn-off shotguns and still hit you at ridiculous distances, beyond the usual range of their weapons. They can accomplish this whilst strafing - meanwhile, you're having fun missing shots that should have hit with your makeshift sniper, whilst crouching and stationary.
ARMA 2 (even with ACE2 and Zeus AI): I'm snaking through tall grass after a hit and run episode. Then the camera pops to third person and there is my corpse, shot by an AKS-74U from 200 metres away. How the hell was I even seen?! I even took out my targets whilst prone, looking through blades of grass.
Minecraft: SSSSSSSSSSSSSS
That's why I play on peaceful when building stuff.
Mount and Blade series: When your friendly troops decide to randomly stop or run in front of you for great justice, whilst you're on horseback. Unlike the enemy, you can't push/knock them aside, leaving you to the mercy of an angry mob or a hail of projectiles.
Thief series: No, those guards didn't even see me. They only heard a little footstep about two rooms away. So as I silently sneak away from them amongst the shadows, WHY THE HELL ARE THEY STILL WANDERING IN MY GENERAL DIRECTION AFTER FIVE MINUTES OF CAREFUL EVASION? I understand that it's to make the player more cautious and the game harder, but still...taffin' psychic AI can go off itself.
SWAT 4: Your team's AI is incompetent as all hell, but the enemy AI...damn...as good as FEAR's.
Street Fighter IV: Seth's second forms on the higher difficulties. Yes it's hard, but there are better ways to go about making a challenging boss rather than a cheap one.
L4D2 single player: "Hey guys, I'm over here just getting my innards torn apart by a hunter. Take your time. Just stand over there ten metres away staring off into space." *whistles whilst guts are flying everywhere*
COD World at War campaign, veteran: Heart of the Reich. The game has a bad tendency of spawning a dozen grenades around you wherever you go. If it's not that, it's you getting shot up through a small gap by the entire Opfor whilst your friendlies ignore all sensible ideas of cover whilst magically being magnetically repulsive to bullets.
FFVI: MagiMaster first time around. Just when I thought I'd won, he casts Ultima on the whole team before dying, effectively killing everyone. That was unforeseen and just so bloody cheap.
IWBTG: "You jumped into a sword. You retard!"
I yelled at the screen then proceeded to keel over, laughing uncontrollably for two minutes.
Mario Kart: Blue shell. How many games punish you for having skill and doing well?