Continuity mistake: When the main character is confronted by her crush 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 the guy, a split second later the cup is on the table next to the CDs, and the CDs are piled differently.
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.
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: 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, 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: 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: 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: 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.
Continuity mistake: In the 'Jingle Bell Rock' scene, right after Gretchen kicks the stereo off the stage, you see her feet, and they are both symmetrical next to each other, then when you see her again one of her feet is behind the other. Gretchen does not move at all between shots.