The optical illusion I'm referring to is perceiving craters as mounds and ravines as ridges. I first noticed this when looking at overhead photos of lunar terrain, or other cratered bodies in the solar system. For example, I might see a lunar crater with a shadow in the lower left (light from lower left) as a mound with the light coming from the upper right. Apparently my brain "expects" the light from a certain direction and gets confused when it isn't.
Perhaps not everyone sees this illusion, but I know I'm not the only one; I've seen photos of cratered terrain accompanied by instructions to rotate the photo if the craters aren't apparent.