MissingNo.

MissingNo., also stylized as Missingno., MissingNO. or MISSINGNO., known in Japan as Ketsuban (けつばん), is the name shared by several glitch Pokémon in Pokémon Red, Blue and Yellow. It was formerly the second-rarest Pokémon to appear from a Poké Ball in Super Smash Flash 2, but was removed in Beta because its function was deemed redundant, along with reports of having scared users playing in full screen mode into mistakenly thinking that their computers had actually crashed.

Pokémon description
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
As stated above, MissingNo. originally could be released from a Poké Ball in Super Smash Flash 2. Upon being summoned, MissingNo. would stand still for a while as it begins to glitch. It would then expand horizontally across the stage and create a fake status screen, parodying the Blue Screen of Death and completely blinding all players. The regular duration of the blue screen is ten seconds.

As with Koffing, CPUs are unaffected by MissingNo.'s effects. Thus, they would attack as if nothing happened, making it a danger to human players. MissingNo. does not affect the pause screen, so the player could pause the game to see where the opponent is.