One of my personal, least-favorite game-design elements is the "monster closet," those in-game tripwires that release enemies suddenly when you pass a certain point, even though the enemies clearly hadn't existed a few moments before. Even if the walls feature doors that, presumably, could be swinging open, it still creates a feeling of fakeness to the game-world.
Monster closets were a chief complaint against id's Doom 3, but they will not be in the company's upcoming Rage. Here's a quote from Matt Hooper, leader designer on Rage:
"[In Rage] when you're going to the RC-Bomb base [from the demo] to deal with the Shrouded clan they're doing whatever they're supposed to be doing. If they're supposed to be defending against you coming there, they'll be doing that. If they're just tinkering or having a conversation they'll do that. You will never see [a monster closet.]"
Thank god. Let's hope that puppet-on-a-string style monsters that exist only to jump out and scare you are a thing of the past in every game: Real life doesn't have trigger points.