There was some mission (probably early on) in Splinter Cell: Conviction where you have to break into this mansion and promptly give some guy a frontal lobotomy with your bullets.
I don't know if it was just because I failed massively at this game, or it was the 'Realistic' difficulty fucking me over, but it seemed that every time I tried to get through this level in a sneaky fashion, the game would spawn an enemy right behind me without me noticing what it was doing. Either that or shooting a light out would alert all the guards in the area or my exact location, aswell as spawn 3 new ones.
On the same game: you're running down a corridor, when suddenly these massive doors on the other end of a room open, flooding the area with light. 3 deaths later, I noticed there was a pipe overhanging the room. After being shot to shit twice while attempting to get to the pipe, I finally succeeded...only to find the guards were mysteriously able to see me when I was hanging above them, hidden in the shadows of the ceiling.
And again on Conviction: the war level. Yeah, that was some great creative thinking. "this is a stealth game, so let's arbitrarily force the player into a war shooter for a mission!". Seriously, Conviction's UI doesn't work for combat, especially not when on Realistic difficulty you die with 3 shots or if someone throws a grenade within 10 feet of you. Putting a stealth character, in a stealth game, into a war-based level with an un-silenced machine gun is about as much use as putting an oil rig in a children's play pool. The pool doesn't have enough depth or material for the oil rig to work in efficiently.
After the war level annoyance I decided to stop playing.
Wow that game had a lot of annoyances.