After four years of school, Doris Finsecker's little brother never gets any older.
When Mrs. Sherwood is arguing with Leroy about his homework, she wads up the same sheets of paper twice.
When Leroy storms out of class, the chair he kicks falls over twice between shots.
When the kids spying on the girls break the pipe, the pipe comes off of the wall. In the next shot it's fixed back on.
When Shirley shakes the metal gate after being told she wasn't accepted, the yellow slip in her hand vanishes between shots.
When the father and son are talking in the taxi, his turns of the wheel do not cause the taxi to turn, as revealed by the view out the window, which of course is not real.
While movie covers four years of high school life ending with graduation in 1979 or 1980, scenes set during characters' freshman and sophomore years include scenes with marquees and signs advertising movies and Broadway plays (Starting Over, Dracula, Evita, etc.) that didn't open until their senior year.
While Bruno is talking to his father who is driving the taxi cab, Bruno's father is shown turning the steering wheel left, but the background shows that the car is turning right.
When Hilary is delivering her soliloquy of broken dreams in the abortion doctor's waiting room, her words and the movements of her mouth and lips are out of sync.
Reflected in various people's glasses.
Exalting in Bruno's music during the big street dance number outside the school, Angelo exclaims, "Today, 46th Street, tomorrow Madison Square Garden!" The High School of Performing Arts is on 48th Street not 46th Street.