Fun Stuff - A Pentagram on the Ceiling

As DM, I once had the party enter a room with a pentagram painted on the floor and a succubus contained within it.  The party, knowing the rules of pentagrams, entered the room assuming the succubus could not attack them as long as they did not cross the boundary of the circle around the pentagram.  Despite their careful movement, it attacked them and the party fought and killed the succubus.  Only after the battle, when the party was wondering why the pentagram did not contain the succubus, did they notice the larger pentagram painted on the ceiling above; which was the true container for the demon.