There was a particular room in Dead Space 2 that endlessly spawned enemies. I did not realize they were endlessly spawning until I'd tried to clear the room for like two hours. Died so many times, used all my ammo, etc. Once I realized I could just run through the room, I was so pissed at myself for wasting that much time.
The fact that this part of the game isn't more frequently referenced is a testament to how much of an idiot I am. Everybody else figured it out.