In episode 63 in Pokemon Black and White Rival DestiniesEvolution Exchange Excitement! . Bianca traded her Shelmet for Professor Juniper's Karrablast, and then as soon as they finished trading they let out their pokemon and then they started evolving into Accelgor and Escavalier.
Go to this website if you want to watch it:
Part 1 http://www.animecrazy.net/pokemon-best-wishes-episode-65/
Part 2 http://www.animecrazy.net/pokemon-best-wishes-episode-65/281298
They got the episode mixed up this is supposed to be Episode 63=D