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.
Continuity mistake: After Lisa wakes from her daydream, there's a boy behind her with blonde hair and a light purple shirt. After Ms. Hoover says "Then come back and finish the test", the boy's hair turns blue, and after it zooms in on Lisa's face, there's a boy with brown hair and a blue shirt behind her.