I could see this being exploited easily in certain areas that actually have the room for this, but a mansion could be easily placed over-top of this cave entrance I've done some testing with this, and it could be done with Brickwalls and other exploitable items. I thought to throw this up in a report before, and because of it's ability to potentially be abusive. :) Dunno if it's something that needs fixed or not, but for those rare occasions when the cave doesn't put the walls down right.
Tested with a cave entrance from the wall area, to produce these results.