Revealing mistake: When the Plastics are walking down the hallway together, Cady falls into a trashcan. The trashcan stays upright, but it should tip over from the way she went in, the momentum and her bodyweight. Just a few shots prior to this, a janitor, on the left side of the screen, is seen moving that trash can, so it is apparent that it is not bolted to the ground. (00:44:00)
Other mistake: When Cady gets back her test (the one scored with a 98) from Ms. Norbury, the date on the test is October 14th, 2003. However, that scene is set after winter break. (00:45:30)
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 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: 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)
Audio problem: When Janis shouts at Cady out of Damian's car, Cady says "It's not my fault you're like in love with me, or something." After she says this, her mouth visibly moves saying more words, but no audio is heard. (00:58:40)
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: 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: 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)
Revealing mistake: Regina writes comments about herself in the Burn Book, at the top of the page in red marker. Later, when Mr Duvall has the book, he opens it saying "Why would Regina refer to herself..." and he then looks at the page and reads "As a Fugly Slut?" However, watch as Mr Duvall opens the Burn Book - the page he has it opened at is blank at the top and there is no writing on it at all. (01:00:15 - 01:02:25)
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: 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 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: When Mr. Duvall takes the mic after Cady's speech at the Spring Fling, he steps with his left shoulder forward. In the next shot, his right shoulder is forward. (01:29:25)
Continuity mistake: During the dance scene, the captain of the Mathletes goes up to Cady's friend Janice and starts dancing with her. At first she's smiling, but the next shot shows her with a straight face. (01:31:15)
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.
Visible crew/equipment: Before Cady enters the house, when one of her best friends is watching a movie the reflection of a crew member is visible on the TV.
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.
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.