Optical illusions sometimes seem like artifacts of the graphics and drawing, but sometimes normal objects can seem strange. Look at the bicycle gears and hub below.
(Click to enlarge.) Weirdly, they don't look like they both will fit on the same hub. To my eyes, the center hole of the left gear set looks smaller than the center hole of the right gear.
They are of course the same size.
And they both fit on the hub.
This is a physical example of a classic optical illusion where the size of the outside affects your opinion of the inside. The Exploratorium has a good Adobe Shockwave implementation of this effect. <http://www.exploratorium.edu/exhibits/changingill/> The third illusion is pretty much the same as this one.
What suprised me was that the illusion was just as strong in person with real objects as with a graphic on a screen.