Ugh, that puzzle room in my D&D game was atrocious! Remind me to only throw one or two puzzles at the guys at a time, not a whole room! Only three out of the eight riddles/puzzles were solved. I think I broke a couple of the guys.
Try this one: hang a picture with wire over two hooks in a manner that, if one of those hooks happened to disappear, the painting would fall because it was not supported by the other hook.