Played with PhotoBooth’s animated background replacements. They’re based on a difference key: you move out of the range of your iSight camera and it takes a snapshot of what it sees, then when you move back in frame it recognizes you as the difference, so it can composite you into a new background. It only really works with good lighting and not very busy backgrounds.
I played with it in low light, and most of me must look a lot like the couch and the wall. Even the pupils of my eyes were knocked out, and through them you could see beyond to the roller coaster track. Most of my face was a Paris mask; a Phantom with an Eiffel Tower on his cheek.