I noticed that if you put a -1 in the image property of an object in a room, the room ends up unusable. I was able to overcome this, by not saving the changes, but if someone somehow messed up with this, I believe it would be frustrating, even if we all know we shouldn't put minus values on object.image property.
Just thought I should mention this.
Hehe, quite unlikely to happen but I'll fix it anyway.