Evolution animation glitch
Pressing B at a very specific time when an evolution is about to finish, may display the text "Huh? [ORIGINAL POKéMON] stopped evolving!", but with the picture of the evolution instead.
Afterwards, the Pokémon remains unevolved in the party, and there are no known side effects of trying to evolve it again.
This can be exploited if the Pokémon has the nickname of its evolution, making it appear as if a final stage Pokémon had gone and evolved.
- SloshedMail: Documentation