there was a pretty big one for me when i first got shot through a wall as a Boomer in L4D. i swore he was hacking: "Fucking fucktard just stopped in front of a wall, looked right towards me, and blew my hacked ass APART! Fucking glitch fuck hack fucker!" Teammate: "Dude, that's not a glitch, that's supposed to happen." Me: (awkward silence)
The biggest on of all time for me was in a gamer called Demon Stone, you may not have heard of it. The gist of it is you control a fighter, sorcerer, and rogue who are journeying around together and must constantly switch between the three and grab health potions randomly dropped by enemies (as well as heroism potions, which instantly charge up your "hero" meter and allow you to do a super-powerful special attack that hits the crap out of everything) and keep EVERYONE alive because if one character dies, you lose, period. In the 9th level out of 10, there is a dragon boss that you fight in two parts. At first, he goes and hides on some stalagmites sticking out of a pit of lava, and you have to dodge his fireballs and hit his wings with projectiles while enemies constantly teleport in to screw up your aim. But that's not the bad part. The BAD part is you then have to fight him toe-to-toe in a comparatively narrow corridor. Once you hit him enough (and this is one of the two things that pissed me off so bad), he backs up, breathes a wall of fire in front of him so you can't even hit him with spells or throwing weapons, teleports in yet MORE enemies to fight you, and REGENERATES HIS FUCKING HEALTH. The other thing that pissed me off so bad went hand in hand: the friendly AI. They hog the heroism potions, but they cant use the special attack without you switching to them and making them do it. They also ho the HEALTH potions. It doesn't matter if you're the rogue (runs the fastest) and have a sliver of health, if the fighter (runs the slowest) has so much as a paper cut, he WILL, I repeat, he WILL see the health potion 30 feet to your left as SOON as it shows up and outrun you there from halfway across the room, leaving you to die because you didn't block the damn ORC behind you. After a similar scenario about ten times with that dragon boss, the floor now has a dent in it, and that PS2 controller has a chip of it missing. I think its under the bed.