Continuity mistake: During the scene where Dr. Hibbert is about to reveal the results of the paternity test, Mason Fairbanks is wearing a blue blazer and black pants, but when Dr. Hibbert points at him and says "This guy", he has a light blue sweater and blue pants.
Continuity mistake: After Lisa finishes singing her song during the audition, there are several coloring mistakes in the shot of the audience clapping. Agnes Skinner's face is ghost white, but her hands are yellow. Mayor Quimby's suit is lavender instead of violet/blue, and his hair is light brown instead of medium brown. Helen Lovejoy has blonde hair instead of grey, and is wearing a purple shirt instead of pink sweater over blue shirt, and Reverend Lovejoy has a green shirt instead of pink, and has brown hair instead of black.