From Glitch City Laboratories
In Generations I to III areas which are 'Out of bounds' refer to those which are outside the normal map area, when going 'out of bounds' the player will usually be taken to another route with a different map structure, usually with different music if the player has moved into another town. However in Red/Blue through Walk Through Walls players often use an external device such as GameShark to access areas which have completely undefined data; resulting in a crash. Some people also use Walk Through Walls without one of these devices, by using most of the Glitch City (Red/Blue/Yellow) trick but making his or her last step when jumping on a slope resulting in the player being able to Walk through Walls, as it causes every step to be a jump. Sometimes as an exception to this when using a Walk Through Walls code in Generation II if the game does not crash it may reset itself into a Glitch Dimension or give the 'This Game Pak is designed only for use on the Game Boy Color.' message for Pokémon Crystal.
In Generation IV, if a player goes Out of bounds into undefined map data, the game will not crash and instead state that the player is in a Mystery Zone. An exception is the Japanese versions of Pokémon Heartgold and Soulsilver, where the game denotes unused locations '----', however later translations of the game denote it as any translation of the phrase 'Mystery Zone'. These 'locations' can by accessed via a game-altering device, but surprisingly can be reached without an external device through Tweaking.
- Mystery Zone - The areas corresponding to those which are out of bounds on Pokémon Diamond/Pearl/Platinum.
- Game freeze - What may happen when the game cannot read data or tries to run an invalid function.
- A player using a 'walk anywhere' Action Replay code to move out of bounds
- A player using a Walk Through Walls code on Pokémon Crystal to move out of bounds, giving the 'This gamepak is designed only for use on the Gameboy Color' message