hovering quicksand?

So I saw this recently, and wondered if it was by design (this must have been around forever?) You can dig a pit, and place quicksand in the room above it. Obviously, this puts the guy you're trying to gank in an even bigger pickle, since he stands a chance (higher depending on room choice) of just fleeing down into the put and then coming back up into your quicksand trap of death. Just felt a bit unfair so I thought I'd ask - should that quicksand not fall down into the pit? T