Impossible Objects, a version of an illusion, are objects drawn in 2D, perceived in 3D, but cannot actually be built as we see them. Some hackers have even figured out ways to make these impossible objects look possible in real life. Necker cube on the left, impossible cube on the right.
Necker Cubes. This is an example of two identical necker cubes, the one on the left showing an intermediate object (blue bar) going in "down from the top" while the one on the right shows the object going in "up from the bottom" which shows how the image can change its perspective simply by changing which face (front or back) appears behind the intervening object.