Level up Transform glitch is a natural glitch in Pokémon Red, Blue, and Yellow.In a Trainer battle, if a Pokémon such as Mew uses Transform (or a Pokémon which acquired Transform via Mirror Move/glitch Pokémon) levels up and learns a new move while it is under the transformation of another Pokémon, it will immediately gain its original moves back in battle, even though it is still under the transformation of the opponent and should normally have the opponent's moveset.