I felt like yanking my player's chains, and getting them in trouble, so while they were sacking the keep of a cult known for summoning demons to do their work (well, demons and devils; while it's meaningless in game, it's important mechanically), I wanted to have them free something big and nasty to see what they'd do with it. So I had them stumble across a summoning circle that radiated overpowering evil, and had inside a single ring.
Unfortunately for them, Pit Fiends know both Improved Invisibility and Major Image, so as soon as one of them broke the circle, he was rather impressively bull rushed back. Since everyone had the sense to run like hell, I let them get away, but I'll have to keep illusions in mind next time I want that cool seeming item they have to go out of their way for to detect as magical.