When Cady is imagining attacking Regina, Regina's nails are perfectly manicured before the attack, but when she is seen fighting Cady on the floor her nails are clearly not manicured and her character's leg is suddenly much thicker and manlier (obvious stunt double).
The two Asian girls who fight speak Vietnamese to each other (with an American accent - both actresses are Vietnamese Americans) in the gym scene, but when Gretchen is their friend at the end, Chinese is being spoken. Also, Pak is a Korean name, not Vietnamese or Chinese.
When Cady is being chastised her first day of school, a teacher yells at her in German to sit in her assigned seat, but as Damian reads her schedule as they go to the "back building," he mentions she is taking Spanish, not German.
When the science teacher denies Cady a lavatory pass, the blackboard behind him incorrectly lists "silicone" among a list of elements from the periodic table. The correct element should be silicon, not silicone.
In the behind the scenes footage, the actor that plays the principal Mr Duvall says his character doesn't have a first name, that he just always goes by Mr Duvall. But in the scene where Cady, Gretchen, and Karen are in the principal's office for the Burn Book, one of the teachers comes in and says, "Ron come quick the girls have gone wild!". Therefore, he does have a name - it's Ron. There is also a nameplate on his desk that reads "Ron Duvall".
After the Junior girls start freaking out and beating each other up over the Burn Book, the principal stops them by pulling a fire alarm and setting off the sprinklers, which drenches everyone. Then he orders them all to the gym immediately. When they get there, everyone is dry.
When Cady enters the room where Janis and Damian are watching Friday the 13th Part 2, Damian screams and throws his bowl of popcorn into the air. When they cut back to him, the bowl is full of popcorn again.
When Cady's parents are preparing for her first day of school, her father hands her a brown paper lunch bag. When Cady hugs her mother while her father takes pictures, both hands are empty and the lunch bag has disappeared.
When Regina and Cady are talking on the phone about Gretchen, Regina is eating a donut. At first, she has bitten into the hole of the donut. Later, the inner rim is still intact, and only part of the outside has been eaten.
Cady's make-up at the Halloween party is significantly lighter before she meets Janis and Damien later that night. If it were lighter after the party, we could assume it was from crying over Regina's betrayal, but it actually gets darker as the night progresses.
When the "banished" Cady ultimately eats her lunch in a bathroom stall, the milk carton goes from sealed (its brand facing the camera) to open with straw inserted (brand of milk now not facing camera) when she sits down.
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.
When Cady is in Health class (after Regina has brought the burn book to the principal's attention) the words "transmitted diseases" changes to "sexually transmitted diseases" and then back again between shots.
When Cady walks into the junior girls meeting in the gym, her hair is still wet and frizzy from the sprinklers. A quick cut to Janis and Damien show that Janis's hair is also wet and the shoulders of Damien's blue hoodie are damp. Everyone else in the gym is completely dry.
After all the girls are sent to the gym, Cady can be seen sitting next to the aisle on the bleachers. Sometimes her foot is in the aisle, and in other scene it is in front of her. However, it is inconsistent.
When all the girls are confessing to their lies in the gym, Ms. Norbury asks, "Who here has been called a slut?". The teacher in the blue shirt raises her hand. When you see her behind Ms. Norbury in the next shot, her hand is down.
When Damian leaves Janis at the Spring Fling, and Kevin takes her hand to dance with her, her left hand is off his shoulder in an almost defensive posture. In the next shot, her hand is on his shoulder.
At the beginning of the "Jingle Bell Rock" scene, Mrs. George gets up twice. The first time is a shot from the stage showing her getting out of her seat to move towards the isle. The very next shot is from behind the audience, showing her again getting up from her seat.
When Karen, Gretchen and Cady are discussing Regina's breakup with Aaron Samuels, Karen has her arms crossed over the table. When she says "I thought she dumped him for Shane Oman," her hand is on her cheek.
At the end of Kevin's rap, when the man says, "Thank you Kevin, that's enough," the guy on stage in the green shirt is on Kevin's right. In the next shot, when Kevin says, "Happy holidays, everybody," the guy in the green shirt is on Kevin's left.
With the type of sprinkler systems used in schools (and most other structures besides chemical plants), it is impossible to activate the sprinklers with a pull station. Only extreme heat will activate a sprinkler head, and only the head affected by the heat will release water. The pull station should activate the fire alarm, but there was no audible fire alarm or visit from the Fire Department.
In one of the final scenes where Regina is seen playing lacrosse, she and all the other players are using men's lacrosse sticks. Women's lacrosse has different sticks and does not allow players to use men's sticks.
When Cady throws up in front of Aaron and Regina at her party, the sound of a guy saying "ew" is heard. Since it is a guy's voice, it would have to be Aaron saying it, however Aaron's mouth doesn't move.
Actually it's more of an "ugh" sound and it doesn't require his lips to move.
Mr. Duvall should not have had his hand bandaged for the entire school year. The actor playing Mr. Duvall broke his hand just before shooting began. His scenes were filmed during the weeks his hand was in a cast.
Cady, Gretchen, and Karen were in Mr. Duvall's office while Regina was passing out stuff in the Burn Book. They were in there because Regina accused them of writing about her in the book, not because they were being accused of passing out the papers. Therefore, whoever passed out the papers would, in fact, remain a mystery.
At the beginning of the movie when Cady has Ms. Norbury's class with Janis and Damian, it is homeroom. Many schools have homerooms where a student has a teacher they have at another point in the day with different people in the classroom.
When Janis invites Cady to her art show that weekend, Cady explains that she has to go to Madison with her parents. The next time they are shown at school on a different day, Gretchen confronts Regina about wearing sweatpants on a Monday. The next scene, Gretchen and Karen ask Cady about plans that weekend and she says again that she has to go to Madison with her parents. Because it is Monday, the weekend when she went with her parents to Madison would have already passed.
During the fight after Regina throws the Burn Book pages all over the school, Regina is the only one who isn't fighting with anyone (or scared, like the kid on the phone). She is just watching with a subtle grin on her face. Strange how no one finds this suspicious.
When Regina complains to the principal and tosses copies of the Burn book in the hallway, this is supposedly the day after she discovers the prank where she was convinced to eat high calorie energy bars. Despite gaining weight, she appears as thin as she was before during this scene.