2 in Mass Effect
first on Insanity difficulty, fighting the Krogan Battlemaster on Therum, he takes trillions of bullets to kill, and can kill you with one melee. How is that fair? I only managed to beat him because he glitched and got stuck, and you know there's something wrong when a part is impossible to beat unless there's a random glitch.
second, fighting a thresher maw, when it goes underground and comes back up without warning directly below the mako, killing you instantly. That wouldn't be worth bitching about, but it's not an attack; Bioware didn't equip the Thresher's AI with the ability to judge where you are before coming up.
In Cod4, a grenade lands, like 10 meters away from me, and doesn't just hurt me, but kills me. That, and how Claymore shrapnel can fly around corners.