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.
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.
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 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.
Revealing mistake: When Ms. Norbury spills coffee all over her shirt, you can see the hand that is to the left is pulling her shirt up with her sweater and rapidly lets go.
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.
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.
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.
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 in 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.
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: 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.
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: 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.
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.
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: 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.