Continuity mistake: After the Halloween party, when Cady walks into Damian and Janis' house, Damian gets scared and tosses all of the popcorn out of his bowl. In the next shot of Damian his bowl is full again. (00:29:05)
Continuity mistake: In the scene where Cady is chasing Damien's car and talking to Janis, when Janis says 'he still doesn't want you' two extras in dark clothing walk up a pathway behind Cady. However when the shot cuts back to her, they have totally vanished. Even though they were wearing dark clothing, you'd still be able to have seen them, and there was not enough time between the different shots for them to have walked into the house.
Continuity mistake: Cady breaks up her Spring Fling tiara that she receives. On stage, she breaks it up into many pieces and throws them out into the crowds; it is clear that the amount we see being thrown into the crowds is a major amount of pieces - almost too much from just one tiara. However then when she leaves the stage, we see her holding approximately half of the tiara still. This would not be possible from the aforementioned amount that Cady has already snapped off and thrown into the crowd. (01:25:15)
Continuity mistake: When Cady is confronted by Aaron in her room, she puts her drink on a pile of CDs, the CD pile starts to fall in the background as there is a shot of him, a split second later the cup is on the table next to the CDs, and the CDs are piled differently. (00:58:40)
Continuity mistake: When the principal is introducing Cady to the class, the amount of donuts on the desk changes between shots. In some shots there are two donuts on top of the donut box, and other shots there are no donuts.
Continuity mistake: When the Plastics are explaining to Cady the "rules" at lunch, an extra in a red shirt walks directly in front of the camera, and then a couple of seconds later, she walks in front of the camera again walking the same direction that she was walking the first time she passed the camera.
Continuity mistake: When Cady runs into her teacher on the first day of school and spills coffee all over her, look at Tina Fey's pants. It looks like she is wearing rubber pants when the coffee is spilling down her front. The color of the pants also change during this shot. (00:02:50)
Continuity mistake: When Cady is starting her first day of school and her parents are saying goodbye, she starts to walk up to the high school. Her dad starts to walk away while her mom stays standing there. In the next shot where it shows Cady's face, her mom and dad are still watching her together. (00:01:45)
Continuity mistake: When Cady, Janice and Damian are in class talking about different things, like watching a movie, and Cady tells Janice she can't see her art show because she going to Madison etc., Cady is applying lip gloss. She holds a pocket mirror and in shots of the mirror you can see Cady holding the pink tube of lip gloss in the same hand, then when it cuts to wideshots, the tube isn't in her hand. The pink tube of lip gloss also has no lid on it, yet near the end of the scene a blue long lid appears on the pink tube. (00:49:10)
Continuity mistake: When Regina becomes angry, and writes the horrible message in the burn book about herself to frame Cady and the others, the handwriting changes between shots. Look at "DO NOT TRUST HER" and "She is a fugly slut." - The handwriting changes noticeably, and "fugly slut" changes a lot - she writes it so the "g" and "y" have curled ends, yet it cuts to her sticking her photo in, and the ends to the tails on the "g" and "y" are straight. The line under "fugly slut," and the exclamation mark both change too. (01:00:15 - 01:00:45)
Continuity mistake: While the girls go wild in the hallway, the principal breaks the glass with a bat making the sprinklers come on. When he yells, "All junior girls report to the gymnasium immediately," in the following shots all their shirts are dry.
Continuity mistake: In the scene where Santa comes in with the candy cane gram, as the camera cuts on and off of Cady, you will notice that her hands move from on top of her desk to in her lap a couple of times. (00:34:50)
Continuity mistake: In the wake of the candy-cane grams, Gretchen's arms change from above her shoulders to on the desk. (00:35:00)
Continuity mistake: In the winter talent show scene, you see Gretchen go to kick the boom box in hopes of fixing the skipping CD, but when she kicks it you see it start to just fall off the stage. Yet in the next shot the CD player shoots forward and hits Gretchen's boyfriend, as if she had kicked it a lot harder than she actually did.
Continuity mistake: On Cady's first day of school, she introduces herself to a student who she thinks is the teacher. If you look directly behind them, there is a girl with a white sweater and blonde hair walking in front of the blackboard who almost reaches the end. The shot changes and the girl is farther away, then it happens again in the next shot. (00:02:10)
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)
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.