|Please note that this glitch only exists in the Japanese versions of the game, or is otherwise a glitch from a Pokémon game which was only released in Japan.
Map F121 is a glitch map in the Japanese versions of Pokémon Gold and Silver. In Japanese Crystal, there is also a non-freezing equivalent glitch map.
It can be accessed by activating the GameShark codes 01F13ED0, 01213FD0 and entering through a door, but it should also be possible to access the map with arbitrary code execution.
It is a location which (according to the Pokégear) is in Cherrygrove City, with the Violet City/Olivine City theme playing.
Via walk through walls, after making one step north or west may reset the game in a Glitch Dimension.
If the player steps down enough via walk through walls, it may take them to an unknown other map in Pewter Gym with the Pokémon Gym theme playing (which may actually be Brock's gym).
If the player steps right enough via walk though walls, the game may freeze.
It is not possible to Fly from this location, Escape Rope, Teleport or Dig away; trapping the player if they save the game there.