At the party, Shaggy and Scooby came in wearing robot scarecrow costumes. During the song, they no longer have the pumpkin masks and they are getting into the music (ignoring the dangerous robots in the room). A few seconds later, they are in normal clothing and the scarecrow overalls have disappeared as well.
In the immediate previous episode Pompeii and Circumstance (2003), Fred could not drive stick-shift, yet in this episode, the Mystery Machine (which Fred can drive quite well) is depicted as standard transmission. Instead, Shaggy is the one who has trouble with it.