You might have seen this shaded gray squares illusion before. Squares A and B are the same shade of gray. (It was created by Edward H. Adelson, Professor of Vision Science at MIT.
Here's a similar illusion with colored squares. The "blue" tiles on the top face of the left cube are the same color as the "yellow" tiles in the top of the right cube.
Don't take my word for it. Use an image editing program with a eyedropper to see for yourself. I used Photoshop's eyedropper tool to take 5x5 samples and found that both the "yellow" and the "blue" tiles are C:50 M:40 Y:40 K:5.
Take a look at the brown tile in the center of the top face and the yellow tile in the center of the side facing slightly to the left. They're the same color.
UPDATE: The color tile illusion is one of many excellent illusions created by R. Beau Lotto.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects