Continuity mistake: Clare's pants split at her knee because she falls off her bike and she gets injured. When she returns home later to see her father, the pants have been fixed and there is no hole.
Continuity mistake: Late in the film, Clare becomes unpopular again and a girl throws a drink at her shirt. The stain the drink makes changes considerably when she is cleaning it in the bathroom mirror (it is darker and more messy). The stain then is back to what it was earlier after that (before we see her wish for her mother to be alive).
