Continuity mistake: The licence plate of the taxi which hits Sebastian in the end changes during the shot. (01:27:35)
Continuity mistake: In the final scene where Annette is driving the Jaguar and having flashbacks, watch as she drives past lovely rolling hills and pastures. Cut to a helicopter shot of her driving the car along a New York freeway (FDR I think...) This mistake has been confirmed by the director. (01:33:35)
Continuity mistake: When Sebastian gives Kathryn the magazine so she can read the article, it is folded in the middle. when you see what she is reading, the page is smooth. (00:12:40)
Continuity mistake: When Kathyrn is in the bathroom (right before the service) she puts some memocards in front of her on the sink. There's a word highlighted with an orange marker. In the next shot the card has got no marked text. The third shot it's back again. (01:29:45)
Continuity mistake: When Kathyrn and Sebastian are arguing because he won't sleep with her, he grabs the phone from her and slams it down. But when the camera shows the desk, the phone is on the opposite side from where he slammed it into the receiver. (01:12:50)
Continuity mistake: When Dr. Greenbaum hands Sebastian a copy of her book, in one shot she's holding the left side of the book, but in the next shot she's holding the right side of the book. (00:03:00)
Continuity mistake: When Sebastian goes to visit Blaine, Blaine is using a scale to weigh out some marijuana. In one shot, there is a big pile on the scale, the camera cuts away. When the camera comes back to Blaine, there is a much smaller pile of marijuana. The camera cuts away and back again...the big pile has reappeared. (00:20:30)
Continuity mistake: In the final scene where Annette is driving Sebastian's car, in a close up shot she is wearing a watch, but in the next overhead shot, she is not. (01:33:30)
Continuity mistake: When Ronald tells Cecile their hours up, he hands her her cello bow, and in one shot he's holding the bow upright, but in the next shot as Cecile takes it, he's holding it sideways. (00:19:05)
Continuity mistake: When Kathryn and Cecile are having a picnic in the park, in a wideshot there's a blue wine bottle and a glass beside Cecile. In the next wide shot the bottle and the glass have moved behind Kathryn. (00:21:40)
Continuity mistake: In the scene where Kathryn teaches Cecile how to kiss, the green clip in Cecile's hair keeps changing from almost fully covered by her hair to visible, throughout the scene. (00:22:30)
Continuity mistake: Near the end when Kathryn is handed a copy of Sebastian's journal you can see as she is reading it that she has nothing around her wrist, however when the headmaster comes out he pulls out her cross and it is twisted several times round her wrist. (01:32:05 - 01:33:00)
Continuity mistake: When Cecile and Kathryn are talking privately the position of Cecile's hair alternates position from behind her ear to covering her ear. (00:53:45)
Continuity mistake: When Sebastian "catches" Blaine and Greg, watch when he says "After all, it is you who is bad mouthing me to Annette Hardgrove". In shots of Sebastian's face, his hand is tipping Greg's chin upwards. In shots of Greg's face, Sebastian's hand is nowhere near his face.
Continuity mistake: When Sebastian is breaking up with Annette, her pink cardigan is pulled back and the top of her bra is being revealed above her dress. In the next shot, the dress is pulled up and the cardigan pulled over.
Continuity mistake: When Kathryn takes the binoculars from Sebastian, you can clearly see her watch down her wrist. However, in the next shot of her holding the binoculars, her watch is all the way up to the top of her wrist. She never put the binoculars down or moved either. (00:57:45)
Continuity mistake: In the end when Kathryn receives a copy of Sebastian's journal, the title page reads, "Cruel Intentions". When several moments later, just before the headmaster takes the crucifix and opens it, the camera pans across the journal in her hands, the title now reads "Cruel Inventions".
Continuity mistake: When Sebastian is explaining to Kathryn that he had the chance to sleep with Annette but he didn't, he is lying on the bed with Kathryn straddling him. Then he gets up and pushes her over. When the camera switches to a different view of her, her legs and arms are in different positions than they were before. (01:05:00)
Continuity mistake: When Sebastian lies to Annette and says he doesn't love her, he has a tear streak run down his cheek which vanishes during their conversation. (01:13:20)
Answer: Well, this is my own opinion, but I think that Annette and Katharine were just pretending not to know each other. Remember, Katharine doesn't know how much Annette knows and Annette knows what kind of deception Katharine is constantly pulling off. I also think that Annette uses the line "turning to God" to foreshadow (and maybe scare) Katharine. Naturally, it didn't work until the journal got out.