At the beginning of the movie, when Julie and Hannah are discussing the lunch table arrangement, the fountain is in a pool of water and the stairs behind are in shadow. Moments later when Steve jumps the fountain, there isn't any pool of water and the stairs are in full sun.
When the security guard car is hit by the little green car, the air-bag goes off. A few minutes later, the guard accidentally drives the same car into some garbage cans, and the same air-bag goes off again.
While walking down the hallway, a girl shows Julie an invitation to Liz's party. Julie takes the invitation using her right hand. In the close-up of the invitation, Julie is holding it with her left hand.
The girls are lying down pretending to be asleep, before the parents open the door Yancey's pillow is right next to Julie's, but when they open the door Yancey's pillow is a whole foot away from Julie's.
When the parents check in on the girls and open the door, Yancy's pillow has colorful polka dots and is close to Hannah's. After the camera goes to the parents faces and then back to the girls, you see that Yancy's pillow has colorful stripes instead and is far away from Hannah's.
At the beginning of the film, the girls are leaving middle school on their last day and talking about the sleepover. Julie is handed a copy of Liz's party invite and is told that Stacy is now bailing on her party. When Julie decides to invite Yancey instead, she hands her the card saying she's now invited to her (Julie's) sleepover. When they walk down the steps and run into "Spongebob", he takes the invite and reads it as if it's Julie's invite.
When asked why Liz can drive already, someone responds that she has her learner's permit because she was held back a year. However, people with Learner's permits are only allowed to drive with a parent/guardian or other adult in the car and she is shown driving her three friends, without an adult.
When the green mini hits the Patrol-Tec security vehicle on the left side door, the driver's side steering wheel mounted air-bag goes off. The mechanism that triggers that air-bag is in the front bumper, not in the door. A side impact would trigger side curtain air-bags, if the car was so equipped, which this model is not.
After Julie drops her crown after the dance and the security guard hits it on his bike, he flips over, and when he gets up and screams at the car that drives by him, you can see his breath in the air. However, this is all supposedly taking place in June.