Continuity mistake: Julie doesn't come home one night; the next day as she is getting out of the pool we see that she has been "roughed up" and has a black eye. Next to her eye there is a circular bruise/mark. That evening, as Julie and Sarah are at dinner the bruise is now under Julie's eye and is a different shape (longer and more oval in shape).
Continuity mistake: As Sarah Morton copies the text from Julie's diary, there is a picture of Julie's mother sitting on the crease of the diary between the two opened pages. In the next shot, the picture is sitting to one side of the diary, yet Sarah has never stopped typing out the diary entries and therefore could not have moved the picture.
Audio problem: The first time Sarah goes to bed and is disturbed by the erotic noises coming from downstairs, she puts in earplugs. We then hear the lovemaking at a significantly muted level, as if hearing it from Sarah's (earplugged) point of view. Next she switches off the light and adjusts the covers, which we hear at a normal volume, but the noises from downstairs are still muted.