Old man glitch
It is a side effect of the Missingno. bug, where the amount of the sixth item in a player's bag, not necessarily a Rare Candy, is set to 127. By using all of these but one and repeating the bug, the player can have an infinite amount of that item.
As the trainer will often encounter Pokémon over level 100 that are hard to escape from while executing the Missingno. glitch, it is a good idea to clone Poké Dolls.
Performing the Glitch
- A Pokémon that knows Fly.
- A Pokémon that knows Surf.
- Party members level 50 or higher (varies)
- At least one of the item you want to clone (for the purposes of this explanation, we shall use a Rare Candy.)
- Talk to the Old Man located North of Viridian City. Allow him to demonstrate how to catch a Pokémon.
- Once he is finished, Fly to Cinnabar Island.
- Make sure that the Rare Candy is in the sixth slot of the bag.
- Surf up and down along the east coast of the 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 your name will cause a Missingno. encounter). (If you encounter three different non-Missingno. non-'M Pokemon along the coast, then the chances are good that your name can't cause a Missingno. battle)
- Defeat, run, or catch Missingno..
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 you have (? being a glitch character).
The glitch can be done both with Rare Candies and with any other item. The most useful items to copy are Master Balls, Nuggets, expensive items such as Full Restores and Revives, or unique TMs such as TM26. Key items, such as bicycles, can also be duplicated; it is very difficult to deposit duplicated key items. Even Fossils and Old Amber can be duplicated, potentially allowing a player to revive limitless quantities of Kabuto/Omanyte and Aerodactyl.
Cause of the Glitch
Every Pokemon has a "Pokedex byte" that tells the game whether you've seen or caught the Pokemon. 0 means you haven't seen or caught the Pokemon; 128 (or 127) means you've seen the Pokemon; and 255 means you've caught the Pokemon.
Missingno.'s Pokedex byte is in the same location as the byte that stores how many of the 6th item you have. This is why, when you encounter Missingno., the sixth item slot is set to 128 (or 255 if you capture Missingno.). And the glitch character? That's a result of the game trying to display "12" as one digit (or "25" as one digit).