(Not sure if this quite classifies as a bug, but this still seemed like the best category to put this in.)

Wall hiding is one of the most common exploits in the game, especially in quests. It's just not fun when you join a quest, play for a little bit and then notice your team mates have gone into the walls. The quests are supposed to be challenging, but when you're in a wall, nothing can hurt you, and you can just kill everything from there.