The Celadon looping map trick is a method of obtaining any glitch item in English/non-English European Pokémon Red, Blue, and Yellow
(with the exception of CANCEL/TM55 (0xFF)).
It was documented by TheZZAZZGlitch, and it mainly abuses the fact that the phantom quadrant of Celadon City is particularly easy to navigate, allowing the player to walk freely on a horizontal line covering a wide range of X-coordinates. As the player's X-coordinate represents an item in the player's expanded item pack, this translates to a wide range of items.
The trick does not work as well in the Japanese versions, as the X-coordinate represents a quantity rather than an item in these versions, and the glitch city quadrant, with the Y-coordinate out of bounds, is usually hard to navigate.
First, obtain an Expanded item pack through a method such as the Dry underflow glitch.
The player should then go to Celadon City and open their menu at the bottom row of tiles (at the spot pictured). The player should then open their bag, and swap an item with an ID greater than or equal to 0x33 with a Nugget x1 somewhere lower in the menu.
If the menu temporarily stops responding due to an unterminated name glitch item, tapping B multiple times should enable it again.
The player is now able to walk right and left. The Celadon City map will be able to loop multiple times until X=hex:32 or X=hex:FF. Each right step increases the item ID of the slot where the Nugget was. Each left step decreases the item ID.
When the player receives the item they wish, it should be swapped again with the Nugget x1 (or an item that the player wants to replace). If the player uses Fly or Teleport, they can safely return to a Pokémon Center without getting trapped.
The player can stand at spots in Celadon City further to the left than the picture above to manipulate item 34 (normally the Nugget x1) to values hex:00 and hex:30, swap it to the regular item pack space and then Fly or Teleport away.
The player does not have to use Celadon City for this glitch, and it is possible to use this method to obtain any item with an ID<(map's X boundary-1).
When the menu doesn't respond due to a glitch item name, it is highly recommended that the player presses B (and not A) at a slow pace, because an accidental B press may close the menu, and a single accidental A press if a unterminated name glitch item is selected may cause the game to freeze with a chance of corrupting the save file.