In Generation IV there is a glitch with the GTS that allows Pokémon to evolve by being traded, but without actually trading them. Players must first place a Pokémon that evolves by trading it (with the correct held item, like Metal Coat for Scyther or Electirizer for Electabuzz) onto the GTS. To prevent the deposited Pokémon from being traded away, players should request an impossible trade, such as a level 9 and under Mewtwo.
The player must then search for a Pokémon on the GTS and trade for it. After this has been done, returning to the Pokémon that was placed on the GTS by the player and taking it back will cause it to evolve.
Obviously from Bulbapedia.