Someone recently described this effect to me, and it does seem to work.
I put together the above image in Photoshop. It’s based on the classic chessboard illusion.
Looking at the image above – what do you see? Are the diamonds getting progressively darker as you move down?
Actually they are all identical, but the top of each diamond is ever-so slightly lighter than the bottom. When they are stacked like this is gives the illusion that A is lighter than B, which is lighter than C, which is lighter than D.
When in fact A=B=C=D. It actually looks even better if you print it out on a decent photo printer.
Update : Apparently some people can see it, others cannot – for me I can see it on the full sized image, but on the thumbnail I cannot.