In one of the classroom scenes, when the camera is on Brandon's face, there's a female student behind him. When the shot is facing the front of the class and Brandon's head, there's a different extra behind him.
When Brandon is telling Brenda about the students in his history class, he mispronounces Steve's name as "Saunders" not "Sanders."
After Brenda dyes her hair, Brandon laughs at her in the bathroom. She punches him and turns to leave the bathroom, turning toward the camera. When she faces the camera, one can clearly see that her hair is in its normal state and not the orange frizzy mess it is supposed to be.
Kelly: That's not a hair do, it's a hair don't.
Steve: Your parents are still together?
Steve: Well look it's not your fault. You've got to stop blaming yourself.
The Policy Of Truth by Depeche Mode