In the first episode of season 21: "Homer the Whopper", Homer is cast as the lead in a superhero movie. He is helped by a personal trainer to get him into shape. When the personal trainer leaves, Homer starts eating again. As a result there's a huge problem with continuity.
Right before a screening, the studio executives are telling each other they were able to fix it in post production. Then a clip is shown:
Lenny and Carl start complaining about the movie.
Lenny: "Half the shots he's fat, half the shots he's thin. It's taking me out of the moment..."
Carl: "Who was in charge of continuity on this picture?"
The script supervisor is sitting in the row under them.
Script Supervisor: "I tried to show them the polaroids, they wouldn't look"
Carl: "It's your job to get their attention"