Revealing mistake: Regina writes comments about herself in the Burn Book, at the top of the page in red marker. Later, when Mr Duvall has the book, he opens in saying "Why would Regina refer to herself..." and he then looks at the page and reads "As a Fugly Slut?" However, watch as Mr Duvall opens the Burn Book - the page he has it opened at is blank at the top and there is no writing on it at all.
Revealing mistake: When the Plastics are walking down the hallway together, Cady falls into a trashcan. The trashcan stays upright, but it should tip over from the way she went in, the momentum and her bodyweight. Just a few shots prior to this, a janitor, on the left side of the screen, is seen moving that trash can, so it is apparent that it is not bolted to the ground.