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.