Developers use it all the time: easter eggs. Facebook has one too, just try it for yourself:
Press up, up, down, down, left, right, left, right, B, A, Enter key, then right click and finally press up, down. Sounds a bit complicated but it actually works, I have tried it for myself… Some magic circles will appear while scrolling. To get rid of them, just refresh the page.
Try & Share… ;-)