Old man glitch

The Old man glitch (known in some circles as the Rare Candy 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; a side effect of this glitch allows the player to duplicate items in his bag. 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.

Mechanics

Results

The sixth item in the bag is duplicated. The quantity of this item will be reset to 128 or 255. After using all duplicated items and then repeating the glitch, an infinite amount of items can be obtained. Key items can be duplicated, but the only key items which are practical to duplicate are the Dome Fossil, Helix Fossil and Old Amber.

Cause

Every Pokémon has a "Pokédex byte" that tells the game whether it has been seen or caught. A value of 0 means the Pokémon hasn't been seen or caught, 128 means it's been seen, and 255 means it's been caught.

Missingno.'s Pokédex byte is in the same location as the byte that stores how many of the 6th item is in the bag, as well as 'M's. This is why, when Missingno. or 'M is encountered, the sixth item slot is set to 128 (or 255 if it is captured). The glitchy box symbol is a result of the game attempting to display a number greater than 99, which causes it to grab sprites from beyond the number sprites. Sometimes it can appear to be a blank tile, but if one goes somewhere else or out of battle it will revert back to an unusual tile.

A way to tell whether " 9" is in fact 9 or [blank tile]9 is to select Toss. The quantity is displayed with a leading zero if the amount is actually 9, and as simply " 9" if not.

Performing the glitch

Requirements

• A Pokémon that knows Fly (assuming the player has the badge to use it)
• A Pokémon that knows Surf (assuming the player has the badge to use it)
• At least one of the item to be cloned (for the purposes of this explanation, the article shall use a Rare Candy)
• At least five other items in the bag
• Access to Cinnabar Island or Fuchsia City

Instructions

1. Talk to the old man located north of Viridian City. Allow him to demonstrate how to catch a Pokémon.
2. Once he is finished, immediately use Fly to travel to either Cinnabar Island or Fuchsia City and then Surf to Seafoam Islands.
3. Make sure that the Rare Candy is in the sixth slot of the bag.
4. Surf up and down along the east coast of either island where the water is touching the land. Whether Missingno. can be encountered depends on the player's name; other Pokémon may appear instead. Continue Surfing along the coast until Missingno. appears (assuming that the player's entered name will cause a Missingno. encounter). Some names may not cause Missingno. to appear. If so, the player may find 'M, which works just as well.
5. Defeat, run from, or catch Missingno. or 'M.

After the battle, open the menu and view the items currently in the bag. If done correctly, the game should show "?8" or "?5" as the amount of Rare Candies in the bag (? being a glitch character).

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