Graphic designers, you should probably stay away from this one. This optical illusion messes with your vision in a way that creates false colours that could linger for over three months - though don't worry, you have to be trying pretty hard to have that pronounced an effect. It's called the McCollough effect, and here is how it works.
Have you ever been on one of those dated websites with bright green or pink text on black, and found that reading content on there has effected your vision when you go to less obnoxious black-on-white text sites? This is an example of the McCollough effect, and one that effected many people back when word processing was done in green on black text.
Here's how to do it to yourself (if you dare). The below images are used to induce the effect - look at the left picture for a few seconds, then switch to the right, then back and forth for at least three minutes.
Now that you've done that, look at the image below - you'll see an effect where faint reddish colour is visible between the vertical lines, and green between the horizontal lines. You'll probably even see green between the lines of text you're reading right now.
If you want to get the full three months of effect, all you have to do is stare for a full 15 minutes. Fun, right?
The illusion is a classic 'afterimage' effect, like that optical illusion where you stare at a pattern then suddenly see Jesus's face everywhere you look. The same effect works with diagonal lines, and in all sorts of orientations. Interestingly, if you close one eye the effect will only appear in that eye's vision.
Just like many things involving the human brain, we don't know exactly how this effect works, though there are a number of potential explanations. Basically, though, all agree that the orientation of straight, even lines and colour aren't something we'd encounter in nature, hence the trippy effect it has on our vision.