When you look at something, how badly does your brain want to establish a lighting vector? Very, very badly, as the Adelson checker-shadow illusion demonstrates:
Dr. Edward H. Adelson is a professor at MIT, and came up with this illusion 12 years ago (so it’s been around a while, but it’s so uncanny I find it worth coming back to anyway). More here, including proof, explanation, and more illusions.
I always liked optical illusions all right. Oh look, those lines are straight, but they don’t look it. It’s a vase; now it’s two people; now it’s a vase; now it’s two people. La, la, la. But this one? Wow. I harbor suspicions of being screwed with even when it’s being explained to me.
Here’s another in the same vein that I hadn’t seen before tonight. The central square on top (dark brown) and the central square on the front side (light orange) are the same color: