The infamous Missingno glitch can be triggered by the player having Pokemon knowing the moves "Fly" and "Surf", going to Viridian City and triggering the Old Man's catching tutorial. Afterwards the player must Fly immediately to Cinnabar Island and surf along its eastern coastline without moving away from it or back onto the island itself, eventually triggering an encounter with Missingno (Or a similar glitch Pokemon named 'M), after which the item in the player's 6th Item slot will be duplicated to a maximum of 255.

This glitch is triggered by the game's internal data replacing the player's chosen name with the "Old Man" string for the sake of the tutorial and it not being properly replaced by another battle. Whatever letters are used in the player's name will also change what form Missingno will take. For example, a W being used as the 3rd, 5th or 7th letter will trigger the Kabutops Skeleton Form Missingno, an X will trigger the Aerodactyl Skeleton Form and a Y will trigger its Lavender Tower Ghost Form.