Ginger Kids - S9-E11
Continuity mistake: When Stan, Kyle and Kenny are going to Cartman's house to turn him back to normal and get surrounded by the Ginger Kids, Kyle is wearing his pyjamas with a coat over the top. But by the time he and Stan reach the barn to shelter from the attacking Ginger Kids, he's suddenly wearing his normal green trousers.
Chosen answer: Cartman has a knack for getting into places he's not supposed to be. We don't need the details. As for how they worked so quickly, either Cartman worked them mercilessly or it's a cartoon, your pick.
Captain Defenestrator