Missingno.

Missingno.is a glitch that occurs in the 1st Gen Pokemon games. The name stands for "Missing number" and was used as place holder data while the developers were working on the game. Since the game boy allows for 255 slots to be used for Pokemon data, and since only 151 "real" creatures were programmed in, players using various cheats can encounter this glitch due to empty unused slots. This also helps prevent other glitches to occur in game. It's harmless, but is often mistaken to be harmful to the player's game, however it is known to corrupt the player's hall of fame data which scrambles the part of the game that stores this data, by filling it with random bytes that usually originate from other previous events such as Pokemon encounters. Normal gameplay can continue, this glitch is also known to duplicate the player's 6th item which can be very beneficial to the player. Minor VRAM glitches also occur when the player attempts to view it's stats, but can be fixed by viewing the stats of a "real" Pokemon.

Different Forms
Missingno. come in different forms depending on the version and language of the game. The most common one is in the shape of a "L" with random pixels. Some different regions and versions of these games may crash while encountering Missingno., to bypass the crash it's recommended that the player encounter the fossil or ghost type Missingno. As these are valid sprite data in the game, these variations never effect the gameplay in any version or region.

How To Encounter The Glitch
If the player is playing Blue or Red, they may encounter Missingno. by use of the Old Man Trick or the Mew Trick. However in order for Missingno. to occur while using the Old Man Trick, the player must have specific letters or characters in certain parts of their name. (Note: A list of the proper letters and characters in the correct slots can be found on the Old Man Trick page.)