The green circle in the center of this image, which Professor Akiyoshi Kitaoka calls a “watermelon”, appears to sway back and forth and be unstable against the gray background. I personally find this image to be difficult to look at for too long. This one actually makes my eyes hurt!
Another good example of this same effect can be found by viewing the Mona Lisa Moving Face Illusion.
(via Akiyoshi Kitaoka)