Look at this photo for a bit, and see if you can spot it.
It's only been very slightly cropped, so that's not it.
I really like the colours and textures of this wall, and wanted to have the lamp in the top-left corner for balance and interest. (Everything is relative.) But I also like flat picture space, and that lamp placement would create single-point perspective. More simply: you should be able to see the right side of the lamp since it's on the left side of the frame. Some people see reality, I see a problem to be solved.
I wonder if this photo bugs anyone now that I've pointed out that it defies our actual experience of the world.