GTA3, that last mission on the first island (I think), Bomb the Base or something? Where you have to cover 8-ball or what'sisname with your sniper rifle while he runs STRAIGHT UP towards the ship, which is guarded by a total of about a dozen or so dudes with SMGs. Who kill that bald asshole the moment they see him. Which causes you to fail the mission. I have used every cheat in the book, I charged in headfirst guns blazing, and I STILL can't shoot all those dudes before my retard "friend" decides it's a good idea to run headlong towards a couple of guys with automatic weapons.
On Guild Wars: Nightfall, that quest where you have to free Koss from the Kournan prison. Even if my allies manage to NOT attract all the wildlife from the local watering hole (which is pretty much a death sentence), and I manage to steer them away from all the mandragors and whatnot that infest the savannah, then you have to fight multiple groups of enemies that are about lvl18-20 or so IIRC. Which tends to cause at least one or two deaths. At -15% to your stats for every death. And then you have to fight a group that's twice as big, has at least one lvl20 guy and one lvl24 hero. Who keeps spawnkilling me over and over and over again. Seriously, I'm not even standing up and I'm down again. I'm also lvl19, one level from the max. I think I might need to level up my heroes, but I haven't had to actually grind in GW up until now, the various main and side quests levelled me up quickly enough.
StarTopia, the Kasvagorian mission, where you have to take over segments from squatters. But I guess I just need to be EVEN MORE patient, and build up an army of about a hundred or so dudes (which is a LOT, and I'll NEED it) before attacking. Still, that's very annoying.
Rail of War, the jungle mission. The GODDAMN jungle mission.