When Kyle asks Stan to try and do Cartman's voice, Stan has a script in his hand. Next frame Stan has no script, then Kyle gives one to Stan.
At the drive-in theatre, before the premiere of the short film, the mayor is introducing the boys and their film. In the wide shot, Cartman is visible with them. In the medium shot, it's only Stan and Kyle visible.
When Cornwallis sings the line, "Which becomes dinner for the human," his lips fail to move.