Continuity mistake: In the episode "Cat Orgy" when Sheily's boyfriend, Sklar, are playing the song for Sheily while Cartman is hanging on the coat rack you can see of what looks like a picture of Cartman's grandparents hanging on the wall. But when Sheily walks over and flings Cartman up into the ceiling for arguing with her, look at the wall, the picture is completely gone.
Continuity mistake: In the episode with the recorder concert. When Cartman and Kenny come into the hotel room, saying that they've found the "brown noise", Cartman is wearing headphones and holding another pair in his hand. Then he suddenly has two extra pairs - he gives one to Kyle and one to Stan (and is still wearing one himself).
Continuity mistake: In the scene where the ATF fires at the group of people coming out of Mr. Mackey's party, they drop dead. A few shots later when the ATF aims the big gun at the door, the bodies and bullet holes in the house are gone.
Continuity mistake: Just before Cartman sends his kitty out of the window with the photo, there's a photo frame on his desk and a "Cartman" cutout tacked above the desk, and on his bed is his frog toy and his hat. When Cartman walks over to the window with the kitty, all these items have vanished.
Continuity mistake: When Cartman is playing "Wild Wild West" with his toys, he has a female doll tied to a piece of wood on his bedside table. Then after his mom comes in and leaves again, Cartman starts dancing, and suddenly the doll has appeared behind him, and is also off of the wood.
Continuity mistake: At the toy store, Stan shows Cartman the three Chinpokomon he is buying: Lambtron, Roostor, and Shoe. When Stan's mother is at the cash register paying for his toys, though, she has a different Chinpokomon and Pingin. At the bus stop the next day, he has yet another different Chinpokomon.