When Kathryn first shows Sebastian's diary to the camera, the front page says "Cruel Intentions". A short time later, the front page says "Cruel Inventions" (the movie's working title).
The first time the inside of Sebastian's journal is shown, "The Bet" page is opposite the page with Kathryn's picture and the phrase "I'm a bitch". At the end of the movie, Kathryn receives a copy of the journal and opens to "The Bet" page which is now opposite Cecile's page.
Blaine's pile of marijuana changes size between shots.
In the closing scene Kathryn looks up at the students after skimming through the pages of Sebastian's journal and they are all looking at her. They are all standing in a shaded area with absolutely no sunlight on them. Just a second later there's a closeup shot of one of the same students and there's sunlight on his face.
After Valmont returns home from his therapist, while talking to Katherine, sits down on the couch and loosens his necktie almost completely. In the next shot it is shown almost completely tied again.
When Bunny is arguing with Ronald, she remarks, "My husband and I gave money to Colin Powell!" Colin Powell has never run for public office to accept contributions for.
When Annette meets up with Sebastian to have a swim the towel she places on the rail is black but when she gets out of the pool she grabs something white which is assumed to be the towel. In fact, the white thing is her bathrobe; before the swim, she places the black towel on the rail and takes off her robe, which is also placed on the same rail .
When Annette and Sebastian are in the pool talking, a blocking cross is visible at the bottom of the pool, which both stand on at some point in the scene.
When Sebastian goes up to the glass in his therapist's office you can see marks from when he hit his forehead against it in previous takes
When Sebastian drives home after going to lunch with Clorissa, he makes a U-turn which forces the taxis behind to stop. However, they start honking when his car goes right - this was obviously planned because there was no reason to assume he would U-turn.
When arriving at Annette's apartment to deliver the package, the doorbell rings before Sebastian touches it.
Cameraman and crane visible in the front windshield of Sebastian's car while driving with Annette.
The camera operator and camera are reflected in the windshield of the Jaguar as Annette drives out of town.
During the "up and down, back and forth" scene in the park, you can see the light screen reflected on the sunglasses.
When they are in the New York Subway, we can see a Los Angeles Metro train in the background.
Sebastian's Journal has the word "friendship" spelled as "freindship".
Cecile's cello teacher says "OK now let's try the G major scale. Once again. And remember, the 3rd note is sharp". The G major scale is G-A-B-C-D-E-F#-G and the 3rd note is not sharp (#).
When Dr. Greenbaum takes a note on billing for the book, a line is visible about Sebastian verging on psychopathic but it is misspelled as 'pyshopathic'.
Sebastian tells Kathryn about what has happened "between Annette and I." The correct grammar is "between Annette and me."