Factual error: The film takes place in a private boarding school the 1950's, however one of the students is studying from a 1980's chemistry textbook. The obvious giveaway is that the spine of one book shows HBJ as the publisher. Jovanovich was not added to Harcourt Brace until the 1970s.
Continuity mistake: Before going to the theatre to watch Neil's performance, the boys are grooming themselves at the school. Todd has combed his hair backwards and styling remedies has been applied. But when the group are about to leave the theatre after the play and Neil and his father passes them at the entrance, Todd has his regular hairdo again.
Audio problem: In the scene where Todd is in the principal's office with his parents and the principal, and is is being asked to sign the paper, during the shot of Todd looking at the paper, his father's voice says," I've had enough, just sign it", and it is very obvious that line was added in because it sounded like it was very distant or over a radio or something.
Continuity mistake: When Knox is trying to talk Chris into going to the play with him, for about four shots between the two of them it changes from snowing pretty hard to not snowing at all.
Audio problem: In the classroom when Keating says, "Armies of academics going forth measuring poetry.", it is obvious that this is dubbed over because his mouth doesn't move correctly with the words.
Audio problem: When Chet is pummeling Knox, Chris pulls him off yelling "Stop it!", but her mouth doesn't move.
Character mistake: Meek's first name is spelled in the credits as "Steven"; however, the letter being signed by the students accusing Keating of misconduct shows the spelling as "Stephen."
Continuity mistake: During the final scene when the boys stand up on their desks, the boy with dark brown hair stands up twice. After Pitts stands up we see him contemplating, then when Meeks stands up he is visible standing up already, but then seconds later he stands up again.
Continuity mistake: The first day of school in Neil Perry's boarding room. Charlie Dalton tells Neil "rumor has it, you did summer school." Neil replies "yep, Chemistry." When the chemistry teacher is handing out assignments and textbooks, Neil can be seen sitting in the class. He should have already completed chemistry.