Other mistake: In the scene where Nicole opens her locker and a note from David falls out, the handwriting is exactly the same as the seen where Nicole leaves a note taped to the bedroom door for her father. Although the letters are larger, the same applies to the note on the father's vandalized car, which says he's popped both of their cherries.
Plot hole: At the end of the movie where David and his friends are attempting to break into Steve's house, David has the door code and attempts to gain access once by using it. Steve then tells his wife and the kids to go hide upstairs and turn off all the lights so nobody will know where they are. Then Steve and his wife go to another part of the house, away from the front door. Then, we hear David and his friends using axes and a big log to try and break down the door, "before* Steve secures the door with household appliances later on. Why wouldn't David have just kept trying the code?
Continuity mistake: At the end scene when Mark Wahlberg and his friend are attacking the girls in the bedroom, keep an eye on Reese Witherspoon's left shoulder. Her shirt repeatedly changes from torn and her bra strap showing to being intact and the strap covered up quite a few times within the scene.
Other mistake: In the scene where Gary and Nicole are leaving school and Nicole is telling Gary "please don't tell Margo'' when we first see them they are wearing clothes that were worn in the ending then it switches. Keep eyes on Nicole at first her clothes is the black dress with pink shirt underneath and her hair is down, it switches and she's wearing red pants, purple shirt, and hair in a pony tail.