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.
When I first saw the photo, I thought the twist was enhanced by the Zöllner illusion, which I’ve posted about before. If that was so, the long shadow edges on the wall would appear to lean over even more, thanks to an effect from the horizontal masonry lines that run across them. But as shown below, the masonry lines, added to the left, don’t seem to affect the orientation of the shadow lines at all. I’m not sure why not.