Factual error: The mall that the plastics state they are going to, Old Orchard as stated by Cady, is an outside mall. However, the mall they go to is an indoor mall.
Continuity mistake: In the Spring Fling scene, when Cady and Aaron are dancing, it alternates shots between Cady's face and Aaron's. Her arm is on his shoulder in one shot and the next shot it is on his arm. This happens a couple of times in the same section of that scene. Really obvious.
Continuity mistake: When the girls are rioting a boy gets on his mobile phone asking his mother to come and get him because he is frightened. The same girl in the stripy top and denim skirt runs down the stairs behind him twice within a few seconds.
Continuity mistake: At the start of the film as Cady's Dad gives Cady her lunch bag, he has nothing round his neck. But in the next shot the camera is around his neck. (00:00:35)
Continuity mistake: When Cady enters the dance after the Mathletes competition, her sleeve is down past her arms and we cannot see her hand. In the next shot, the sleeve is rolled up and her hand is visible.
Continuity mistake: On Cady's first day at her new school, she walks toward the front entrance through a crowd of kids. A black girl with a green comb in her hair approaches Cady from the right and passes without incident. Yet in the very next shot, the same girl again approaches Cady and purposely bumps into her. (00:01:50)
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)
Other mistake: When the girls are going wild, the two Vietnamese girls, who made out with Coach Carr, fight and argue. There is a subtitled translation at the bottom of the screen and one of the girls' lines is translated as "Your the slut." This should have read, "You're the slut."
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)
Revealing mistake: In the scene where Mrs George is taking pictures of the girls for the Christmas concert you can see that the camera is turned off.
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)
Factual error: While the plastics are at the mall there is a Bell Mobility booth seen behind them. This booth wouldn't be found in an American mall as they only service Canada.
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)
Deliberate mistake: When Cady is breaking the 'plastic' crown into bits and throwing the pieces to people in the crowd, it's not possible to have that many pieces from that one crown. She must give out at least 7 or 8 big chunks, which just doesn't truly reflect the size of the crown.
Visible crew/equipment: When Damien and Janis drive off leaving Cady on the sidewalk with a picture, a large square stage light on the set is reflected in the window of the back car door (probably one which lights the ground to give the effect of a streetlight). It is too large and bright to be one of the lights from a house across the street.
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)
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.