Missingno. and it's twin, 'M Block
PUT THEM IN YOUR PC BOX, AND TOGETHER, THEY WILL ANNIHILATE YOUR SAVE FILE!!! /shot/
Yeeaah. They're somewhat scary if you accidentally put 'M block in your PC box w/o giving it a piece of rare candy, then it kills your save file randomly. Without you expecting it.
Pokemon (c) Nintendo/Game Freak/Creatures Inc.