From the game Metroid II for the Game Boy
While falling on a screen that scrolls vertically, press select as fast as you can. If you press it fast enough, then the screen will start showing holes in the walls. If you go through these holes, then you may end up in a glitch world. When there, there will be random pictures of enemies and the ground pieces. This also works if you are walking horizontally, but it's easier if you're falling.
Submitted by: Glitched Symbols