In the image at the right the square that is labeled A is the same color as the square that is labeled B. I checked this in an image program and they have an RGB hex value of #787878. The website for this illusion photo explains this as follows: "In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison." So I guess this means don't always believe what your eyes tell you.

Head over and view a larger photo of the illusion and the full explaination.