MissingNo.
Even though it has only appeared in three games in Pokémon, MissingNo. has had a lot of impact. Nintendo says it a "programming quirk" and doesn't want players to encounter it and later making impossible to obtain it in the next game, Pokémon Yellow, unless a trade is done. However, information on how to encounter MissingNo. was printed in a lot of magazines and player's guides due to its effect, while some Pokémon players tried to sell real and fake "tips" for capturing MissingNo. for up to $200. In 2009, IGN put MissingNo in their "Top 10 Easter Eggs" in games list, describing it as a "glitchimon" and saying it was useful for making copies of items. In another article, they said "It really says something about Pokémon fans that they took what is a potentially game ruining glitch and used it as a shortcut to level up their Pokémon."