What happens is it you build too far over on a ledge, the buildable itself, hitbox and all, will fall. When it has fallen, it is no longer on the ledge, so you can now build something else there. The new buildable falls, and you get hours of fun.
I once had a dozen acid tubes stacked on each other on highrise, but the models fell, so it looked like just one tube. People who got too close to the tube got a pretty big surprise.