You already know that looking at a bright light or a funky pattern might leave you with an aftereffect on your eyes similar to what you just looked at for a few minutes. But how about looking at something which can cause an aftereffect on your brain lasting up to three months? This is the McCollough Effect.
Seriously. Don't do this if you work with text or video.
Tom Scott over at Things You May Not Know on YouTube talks about the brain's ability to get something stuck in it like an obnoxious earworm song for example. The McCollough Effect is much more serious, however.
It's the idea that by looking at a red and green barred image, followed by a black and white barred image, your brain can be rewired to see that pink hue-like aftereffect for up to three months if viewed for a period of 15 minutes or more.