MissingNo.

MissingNo., known in Japan as Ketsuban (けつばん) is a name shared by several glitch Pokémon in Pokémon Red, Blue, and Yellow. It is one of the rarest Pokémon to appear from Poké Ball in Super Smash Flash 2.

Pokémon description
The name is most commonly used to refer to a Normal/Ghost-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
As stated above, MissingNo. can be released from a Poké Ball in Super Smash Flash 2. Upon being summoned, MissingNo. will expand horizontally and create a fake status screen, obviously parodying the Blue Screen of Death, completely blinding all players. The regular length the blue screen is of ten seconds, but may vary depending the situation, such as lag produced by the computer or playing in Training Mode at speed lower than x1. It should be noted that MissingNo. has a cry that sounds like Windows error alarm.

Like Koffing, CPUs are unaffected by MissingNo.'s effects. So they attack as if nothing happened, making it a danger to human players.