Fiend is Like Friend Without the 'R' - S1-E4
Trivia: Nergal is the villain introduced in this episode. "Nergal" originally was the name of the Babylonian God of Fire and Guardian of the Underworld. In the Bible, he is one of the demons allied with Satan himself. In the series Nergal is associated with the underworld and fire.
Little Rock of Horror / Dream a Little Dream - S1-E13
Continuity mistake: In Dream a Little Dream when it shows Grim, Billy and Mandy eating the pizza, the box is empty. But when they wake up, there is one slice left.
Hoss Delgado: Give it to us straight, Goodburger. Is our little Remus.
Mandy: Billy.
Hoss Delgado: ...Billy, on the bullet train to Smartsville, or is he riding the slow pony to the rubber forest?