This ad created for The Economist is designed with a motion sensor which lights up the bulb whenever somebody walks directly underneath it. Which could be disastrous. What if somebody walks underneath exactly as they're having a terrible idea?
Sugar free Chupa Chups Great ad- completely solved it with a visual and a tagline. It's so simple but really clever. This is the type of thing that I'd like to make. There's a ton of other great ads in this article, definitely worth checking out.