I've noticed mobs falling off things since U9 update. Not sure if it has always been like this and I just didnt notice. It has happened in a lot of different places, a few I remember:
In dq1 the genies around the bridges can fall off into the lava. They will warp back up to the point where they fell off and then drop back down into the lava in an endless loop.
In Claw I have seen the second guardian drop down to the lower level with the first guardian.
In ToD have seen tieflings fall into the lava.