At the assembly, Principal Duvall says he won't cancel the dance because they already paid the DJ, but when it gets to the dance scene there is a band.
During the party at Cady's house, Karen hands Cady a shot and the color of the plastic shot glass changes between angles.
During the scene of Regina asking what percentage 48 calories from fat out of 120 total calories is, Cady answers correctly but with the wrong proof of saying 42 over 120 is equal to x over 100 and cross multiply which would yield 35% instead of the correct 40%.
During the cafeteria scene an equipment truck can be seen driving by outside the window.
When Cady is sitting with the plastics at lunch for the first time, Regina asks what percentage is 48 out of 120. Cady solves the equation and says 42 over 120. Which is ironic since part of the movie is about how good Cady is at math.
When Damian is telling Cady about what happened in middle school, the rainbow button on Cady's plush unicorn is upside down. The red stripe should be on top.