MissingNo.

MissingNo., also stylized as Missingno., MissingNO., or MISSINGNO. and 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 it was removed as of 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
In the early  video games, the programmers used variables to refer to different Pokémon by number. Variable sizes must be powers of two. In the Generation I games, the smallest variable they were able to use was the size of one byte, which 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" for additional Pokémon. These empty slots are filled with "glitch Pokémon", which are simply collections of data that the game handles as Pokémon species.

Arguably the most well-known glitch Pokémon in the game series is MissingNo., which takes up 39 different slots in Pokémon Red, Blue, and Yellow. Its sprite most commonly consists of glitched pixels, namely a block in a backward-L shape in Pokémon Red and Blue and a more scattered group in Pokémon Yellow. It also has three other forms in Pokémon Red and Blue based on existing sprites in its index number, including a Kabutops Fossil, an Aerodactyl Fossil, and a ghost. Encountering MissingNo. requires the player to perform a glitch pertaining to its index number, and it may lead to a variety of subsequent glitches occurring.

In Super Smash Flash 2
MissingNo. could originally be released from a Poké Ball in Super Smash Flash 2, in which it appeared as its most common form from Pokémon Red and Blue. Upon being summoned, MissingNo. would stand still for a while and begin to glitch. It would then expand horizontally across the stage and create a fake status screen, parodying the "" and completely blinding all players. The regular duration of the blue screen is ten seconds.

As with Koffing, computer players 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 past the status screen.