Continuity mistake: When we see the first shot of Regina throw a pile of Burn Book sheets to the floor, they fall and all spread out. Then she sticks a sheet about Trang Pak up and then in the next shot as she continues to walk, the sheets she first dropped behind her on the floor are all much closer together in a different position. (01:02:35)
Continuity mistake: When Cady comes home from her first day her father sets down his book on the notepad on the desk as he sees her. In the next shot as she goes inside the book has moved back. (00:05:20)
Continuity mistake: As Regina is about to write the things about herself in the Burn Book, as she flips to a new page we can briefly see the page about Miss Norbury on it, and it doesn't say "Ms Norbury is a pusher. A sad old drug pusher," but later on when Miss Norbury picks up a photocopied sheet of the same page, it does have the previous comments written in. (01:00:05 - 01:03:55)
Continuity mistake: When Cady is outside on the field with Janice and Damian supposedly going to the back building, there is a very large shadow in front of the football uprights in the background (behind Cady) which keeps disappearing / reappearing. (00:06:15)
Continuity mistake: When Cady is trying to get out of going to Madison, watch the level of her Dad's glass of water. From the first shot of the table to when he picks it up whilst saying "I think Cady's old enough..." the water level lowers. (00:52:50)
Continuity mistake: At the beginning of the second day of school, Janis jumps from toying with a pencil below her waist to above her shoulder. (00:05:30)
Continuity mistake: Just before Regina gets run over by the bus, watch her hair. It changes from on her shoulder to behind her neck between shots.
Continuity mistake: In the opening shot of Cady in calculus class, as her voice-over says "I was so happy to get to math class", the big textbook sitting on her desk alternates from closed to open. This happens again after Cady says "So Cute..." (00:12:10)
Continuity mistake: Between the first two shots of sex-ed on the second day of school, a key hanging around the coach's neck goes from hanging out of his tracksuit, to inside his tracksuit. (00:06:50)
Continuity mistake: When in Regina's bedroom, Regina takes off her pink cardigan, and puts it on a couch, just before the Plastics examine themselves in the mirror. When Regina's mum brings in drinks for them though, the top is positioned much differently. (00:18:45)
Continuity mistake: When Cady is at the Halloween party her face is painted white/gray. when she comes through the door to see Damian and Janis her face has changed from white/gray to a more green.
Continuity mistake: When Regina is being carried by some boys on the field, her "R" necklace is hanging out of her blue P.E. shirt. When they put her down it is tucked under her shirt, then when she blows a kiss it is back out again. (00:07:40)
Continuity mistake: When the Plastics spot Gretchen's crush, between shots Gretchen goes from having her arm by her side, to playing with her necklace. (00:16:35)
Continuity mistake: Before Regina starts writing the nasty things about herself in the "Burn Book", there is a red pen on the floor near the book. The next shot in a closeup of the book the pen has now moved to being right next to the book. (01:00:05)
Continuity mistake: In the wings after the Jingle Bell Rock dance, Aaron has his arms around Cady and Regina. Just as Kevin leaves we can see Aaron's hand is on Cady's shoulder, but in the next shot it is down by his side.
Continuity mistake: When the Mathletes are entering the spring fling, Cady's hands are in her jacket. When the shot changes, her hands are out of her jacket and can be seen. (01:26:20)
Continuity mistake: In the scene near the end of the movie, where Janis is dancing with K.G., after Janis tells him that she's Lebanese, she smiles. The next shot, she's not smiling.
Continuity mistake: When Regina is asking Cady about her home schooling, Regina's hair starts to fall over her shoulder, then the shot changes and it is back behind her shoulder. (00:11:00)
Continuity mistake: At the Halloween party, Karen tilts her head as she ponders cousin relationships, but in the next shot she's upright again. (00:26:30)
Answer: Regina enjoys manipulating people, and she views Cady as a relatively easy person to manipulate, finding her more useful/amusing to keep in their circle. It's a running theme of the film that the friendships between all of the Plastics is very tenuous, because of the catty, manipulative nature of the girls.
Purple_Girl
Also, if Regina tried to ban Cady, people might think she was threatened by Cady. Better to keep Cady around and attack her in subtle, manipulative ways.