Cartman claims to have never had a sibling, but in the Unaired Pilot (1991) Cartman is shown having a sister.
The notch on Mr. Kitty's ear is sometimes on the left ear, sometimes on the right ear.
When Shelley drags Cartman across the living room in front of Skyler's band, there's no keyboard next to Skyler. When he begins singing "Shelley," it appears.
At 2:24, Liane Cartman tells Shelley not to mind the noises the cat is making because "she's in heat," but the cat is actually a boy.
The reels on Cartman's tape recorder spin in the wrong direction, they should be spinning counterclockwise.