Normally, I can see the logic in an optical illusion, and the novelty of it wears off pretty fast. This one however, has been plaguing my thoughts for the last couple days. If you look at the picture below. The squares A & B, are actually the same colour!! WTF??! Even when you put a strip of grey on top of the image, you can see that the greys are in fact, the same colour.
The explanation can be found in the way our Visual System perceives the world. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view. Logically, you expect the tiles to be differing colours cos of the pattern in the pic.