In 2006, scientists discovered a 3D optical illusion that had never been reported before. Frederick Kingdom, Ali Yoonessi, and Elena Gheorghiu of McGill University noticed that when placed side-by-side, identical images of objects tilted and receding into the background appeared to have different angles.
So while one picture of the Leaning Tower of Pisa looks normal:
Put two copies of the same image next to eachother and suddenly one seems to veer off at an angle:
This effect, reported in Scientific American here, occurs because of the way that our brain constructs a three-dimensional image from two-dimensional input. One of the rules that it employs is that parallel objects (such as train tracks) converge in the distance. When two pictures are placed side-by-side, there are two convergence points, but our brain only wants to see one, so it imagines that the objects are not parallel. This creates the illusion that one of the objects is at a different angle to the other – even when the images used are identical. More information on this illusion and its different effects can be found here.