Other mistake: When the switch is thrown to electrocute Terrence and Phillip, it's pulled downwards. A few shots later, we see Cartman jump in to save T and P by turning the switch off. He also pulls the switch downwards instead of pushing it upwards. (01:06:05)
Continuity mistake: In the very beginning, when the Narrator is singing the very first song she starts to move around the lecturn she is leaning on, it then cuts to another camera and she is behind the lecturn again and starts to move away from it again.
Continuity mistake: When Miss Hannigan, Rooster and Lily are singing "Easy Street" outside in the street, Miss Hannigan beats her fur shawl on the ground, and a large piece of fur falls off it. In the next shot, this piece isn't lying on the ground anymore. (00:50:55)