Whenever I'm playing a stealth mission on a game that randomly generates enemy spawns (MW2, evasion), I'm out of bullets in my silenced guns, and there's an enemy looking right where I need to walk and there's no way around him. I can't shoot him and when he sees me he will kill me almost instantly and if I manage to run 9001 dogs pounce on me.
I also hate how in MW2, if a dog is on you, bullet damage turns off but enemies still shoot you, so as soon as you stand back up, you're forgeted because bullet damage turns on and a circle of enemies are around you shooting you.