Here is a vivid real world illusion, spotted and photographed by my friend Eddy Keon:  it’s a brand new wall in Nottingham, UK, and it’s perfectly vertical, but with these shadows, it looks twisted.

The twist seems to be due just to the way the edges of the shadows running down the wall lean over more and more as the eye travels from left to right across the photo.  It’s a kind of crazy perspective effect!