many mobs seem to ignore game objects like buildings, boxes, walls, trees. under the correct circumstances.
some of the things that seem to cause this are.
fear, mobs running away at low hp, patrol paths.
one example of fear is in RFK before the bridges, there is a object wall of vines that blocks the 2 areas which you can see through. if a mob is feared there is a high chance for it to run strait through the wall and pull mobs from the other side that will then run strait through the wall at the players ignoring collision.
one example of mobs running away is a mob in a corner of the pirate house in tanaris turning and walking strait through the wall.
one example of patrol paths is a undead in western plague lands at felstone field which spawns at a lower point to the south of the field. it then walks strait through the ground staying at the same height and will chase players from under the world casting frostbolt.
other notes about game object collision include warriors being able to charge through objects at the correct angles or get stuck inside medium sized boxes if a mob is standing directly on the other side of the box.