Glitch

Bulbapedia also has an article about Glitch.

Wikipedia also has an article about Glitch.

See also Glitch (verb), the verb form of the word.

Glitch when used as noun refers to something happening or appearing in a program when it was not the developer's intention for it to work that way, be accessed that way, or be in the program itself. The effects of glitches vary, from simple graphical glitches to strange Pokemon, attacks, or items that were never intended to appear in the game. Glitches may be harmful or harmless to a player's save file.

In the Pokémon community, the term glitch also includes several things that actually aren't "glitches" in the technical sense, like the Pokémon ??????????, which was programmed into the game as a placeholder.