Sure to give a head ache

You messed up the pattern. And that happens with any chessgame, or even a blank board, or almost anything if the wall is in low light!!! Try it. What happens is (Just a theory) after staring at an image for so long, your brain... how do it put this... remembers the image. Really what you are seeing is only a ghost image! Its kinda like getting a song stuck in your head.
The same thing happens to me sometimes after I read. I can see the words. Once I was able to read them!!! Lol
Sorry to ruin the coolness of the thing but it happens all the time, even after a camera flash.