MissingNo.

In the Pokémon series
MISSINGNO., known in Japan as Ketsuban (けつばん) is a name shared by several glitch Pokémon in Pokémon Red, Blue, and Yellow. The name is most commonly used to refer to a Normal/Bird-type glitch Pokémon whose sprite consists of corrupted data. It is arguably the most well known glitch Pokémon in the game series.

In the early Pokémon video games, the programmers had to use variables to refer to different Pokémon by number. Variable sizes must be powers of two. The smallest variable they were able to use was the size of one byte -- that is, capable of holding any value from 0 to 255. (The next smallest size could only hold 0 to 127, which would not have been enough for all 151 Pokémon.) Because 255 is greater than 151, this left several unused "slots". MISSINGNO. and other glitch Pokémon fill these empty slots, with different forms of MISSINGNO. actually using several.

In Super Smash Flash 2
In Super Smash Flash 2, MISSINGNO. makes an appearance. Upon being summoned, MISSINGNO. will expand horizontally and create a fake status screen (obviously parodying the "Blue Screen of Death", completely blinding all players for ten seconds.