Old man glitch

The Old man glitch (not to be confused with the Item duplication glitch) is an infamous glitch in Pokémon Red and Blue. It is one of the ways in which a Trainer can encounter Missingno. and 'M. Its name comes from the old man in Viridian City, whom is required to complete the glitch.

Through the development of Pokémon Yellow, which occurred in the two years following the release of Pokémon Red and Green in Japan, the Old man glitch was disabled by blanking the data for wild Pokémon before overwriting it, and reprogramming shore tiles to act as water tiles. However, even before Yellow, this glitch was fixed in some European versions of Red and Blue, such as the Spanish and Italian versions, but still remains in other European versions, such as the German version.

Performing the glitch

Talk to the Old man located north of Viridian City. Allow him to demonstrate how to catch a Pokémon. Once he is finished, immediately use Fly to travel to Cinnabar Island and then walk up to the east coast. Surf up and down along the east coast of the island where the water is touching the land. Do not to leave the coast. Wild Pokémon appear, based on the player's name.

Chart^[1][2]

Only characters possible to include in the player's name are included.

Cause

The 1-square-wide strip of water on the right side of Cinnabar and Seafoam Islands was programmed to be a place where wild Pokémon can appear, but Gamefreak did not define what Pokémon can appear there. When the Old man demonstrates catching the Weedle, "Your Name" is changed to "Old Man". To do this, it overwrites the variables you gave to "Your Name". Therefore, the game must store the variables that make up your name in another place, so it can recall them later. The game stores your name in the slots that define which wild Pokémon appear. Usually, whenever the player enters a new area, the slots will be rewritten. However, if there are no wild Pokémon in the new area entered, the slots are not rewritten. So if the player flies to Cinnabar Island, where there are no wild Pokémon, then the data is not rewritten. Because the coast contains the code to enable wild Pokémon to appear, the game encounters a Pokémon by misreading the players name as wild Pokémon data. The odd characters are read as levels and the even characters are read as wild Pokémon. It can read up to 7 characters. The Pokémon 'M appears with any name as it reads the byte before the players name, which is always zero. The first byte is the level of 'M. The second byte is the next wild Pokémon, then the next its level, etc.

References

This article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.