Stare at the center of the wheel, and watch what happens in the purple regions when it stops moving.

I recommend viewing on a laptop or desktop; the illusion may not work on small mobile displays.

This is the "Perceptual Tachyons" illusion, by Christopher Tyler. For more information, see his paper (where the image is from) and my blog post on it.

Thanks to Alec Jacobson for help making this page.