I love optical illusions and I found this one the other day while reading up on the new Adobe Lightroom photo editing application. The Checkershadow Illusion shows how you just can’t trust your eyes when working with images. You need to use tools to make sure you get things just right.
The object of the illusion is to show how lighting can make the same color look completely different. in this case the “A” box is the same shade of grey as the “B” box. Pretty hard to believe but it’s true! 
If you like these, there are more examples on the Lightness Perception and Lightness Illusions page.,