Evolving Raichu is a famous text error in Pokémon Red and Blue where an in-game trade NPC claims that the Raichu the player traded to him "went and evolved". A special feature of this text error is that the dialogue line in question is not a single hardcoded piece of text, but a text template used for multiple in-game trade NPCs, which is presumably why this error was not discovered in development.
This NPC can be found in Cinnabar Island's Pokémon Laboratory, and requests a Raichu in exchange for Electrode.
- Walk to this man in Cinnabar Island's Lab. that wants to trade a Raichu for an Electrode. Select Yes and initiate the trade.
- After that, speak to him again, and he will tell you the Raichu evolved.
Why it happens
This error happened in localization: English Pokémon Red and Blue took its in-game trade data from Japanese Pokémon Red and Green, but the text templates of in-game trade NPCs were translated from Pokémon Blue (Japanese). In Japanese Red and Green, none of the Pokémon the player can trade to an NPC would evolve by trading, and thus this text template says something unrelated to evolution. On the other hand, in Japanese Blue, for the two in-game trades using this text template, the player needs to trade in a Kadabra (for this one) and a Haunter (for the other one) respectively, both of which would evolve by trading, so the NPC claiming that they "went and evolved" is appropriate (and may serve as a hint at the game mechanic).
Incidentally, for the other in-game trade using this text template, the player trades in a Poliwhirl for a Jynx, and the NPC will similarly claim that the Poliwhirl "went and evolved". Since the intention of this line in Japanese Blue is likely to hint at the game mechanic, this can also be considered an error, albeit a more ambiguous one since Poliwhirl can evolve to Poliwrath (when exposed to a Water Stone).
- According to Yomiuri Shimbun, Raichu was once in fact originally intended to evolve into a Pokémon known as "Gorochu" with fangs and two horns. It was apparently removed from the game due to balance concerns (source).