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Glitches are unintended behavior in software programs. They can cause various issues ranging from benign, such as graphical and audio glitches, to hazardous, such as complete file corruption or deletion of the save file.
Glitches can be triggered from mistakes within a game's code, or by exploits the programmers didn't anticipate, thus causing the game to react unexpectedly.
The Pokémon games, especially the Generation I titles, contain a number of particularly well-known glitches.
- Main article: Glitch Pokémon
Glitch Pokémon are the result of any bug that causes a game to read special-purpose data or data outside of its internal Pokémon definition list as a Pokémon definition.
- Main article: List of glitch moves
A glitch move is a move not intended to be part of the game, but can be accessed through the use of glitches or cheating device. In Generation I, many glitch moves are named after TMs or HMs; TMs numbered 01 to 55 and HMs numbered 01 to 05 exist as moves; however, some have no name or glitched, unreadable names. Some glitch moves are of known glitch types, but others have either no readable type or an unknown type. Usually, only glitch Pokémon will learn glitch moves.
One way of teaching a Pokémon in Generation I a glitch move is with a Pokémon that can evolve by trading. This can be achieved by trading a trade evolution Pokémon from a Generation I game to a Generation II, at a level where its evolved form will learn a move not in Generation I. Trading the Pokémon back to the Generation I game will cause the move to become a glitch move. For example, trading a level 48 Haunter from Pokémon Red to Pokémon Gold will make the Haunter evolve into Gengar. Since it is level 48, it will learn Mean Look. If it is then traded back to Pokémon Red, it will still have the move, but the game won't recognize it properly since it is a Generation II move, so will become TM12.
Any Pokémon using Transform, or using Transform via Mimic, can learn -- with the transform glitch.
- Main article: List of glitch types
There are different glitch types that are found to be the types of several glitch Pokémon and moves. The majority of them are used for very few Pokémon or moves.
There are several locations which can only be reached by means of glitches or hacks. One of the most well known of these is the Glitch City. Other examples of these are areas in the Sevii Islands that are retrievable via their index number pointer, however, do not have any other data. Sevii Isles 8 and 9 are the only index number areas which have actual map data. There are also other beta locations such as the unused Safari Zone.
- Main article: List of unobtainable items
Placeholder items are often left in the game's code to prevent it from crashing if the data is accessed, such as the Teru-sama item in Generation II. Likewise, other generations have placeholder items, with most appearing at the end of the item list, though a few, likely removed during development, appear in the middle.
Often, placeholder items change into real items (key items or otherwise) in later games of a generation. The GS Ball, Clear Bell, and Egg Ticket, which only appear in Pokémon Crystal, are programmed into the games' internal list where Teru-samas existed in Pokémon Gold and Silver, and will, if they are hacked to be held by a Pokémon being traded to the earlier games, transform.
Thus, glitch items that become real items from a later game only appear in Generation II and Generation IV, as all items not present in Pokémon Ruby and Sapphire that were reintroduced in Pokémon FireRed and LeafGreen and newly-introduced in Pokémon Emerald appear at the end of the list. They would cause a copy of Ruby or Sapphire to crash when it is selected, as they are beyond its item list. Generation II is the only generation in which all new items replace glitch items in the middle of the item list, thus not causing a permanent problem if transferred, as the Griseous Orb is the only item in Generation IV that is programmed at the same index number that a glitch item is present at in Pokémon Diamond and Pearl, while the other items introduced in Pokémon Platinum and reintroduced in Pokémon HeartGold and SoulSilver are after the end of the Diamond and Pearl list, and thus crash the game if the glitch item in their place is selected in a game they do not exist in.
- Main article: Glitch Trainer
Glitch Trainers have been known to occur in Generation I and Generation II. They, like glitch Pokémon, result from the game reading trainer or party information from an area in the game code that does not contain that information.