This image caught my eye while I was at work and the picture quality is a bit bad I know, but this was a very spur of the moment idea. I thought it was pretty good example of turning a set into an illusion. The illusion is supposed to be a close up of a face focusing on an eye and nose. I was thinking that including something like this in the final set would be a good idea because it lends itself to the idea of ambiguity; in one way it is a simple photographed set and in another a close up of a face.
(If you're having trouble seeing the illusion then take the hanging light in the foreground as an eye, the chair in the background as the bottom of the nose and the window to the right as another eye.)