Other mistake: Jen attempts to shoot one of the killers, but the gun is empty. She pulls the trigger several times, and each time you hear the hammer hit an empty chamber. But it is a semi-automatic pistol, and if no round is fired, the gun will not cock. She did not manually cock it between attempts to pull the trigger.
Continuity mistake: When Jen is driving her Volvo convertible, she gets stopped by one of the neighbours. Just before she gets stopped, you can see the back window is up. When the camera shows Jen talking to the neighbour, the window is now down, and when Jen drives away, the window is now back up again.