Continuity mistake: During the presentation the heart on Marie's friends necklace keeps moving positions during different shots. Sometimes it's hanging from the left side of the heart, sometimes the right. It is extremely unlikely that the heart would move that vigorously while she is calmly sitting in a chair.
Continuity mistake: In the very beginning of the movie Marie almost hits Elena with her car in front of the school. After Elena apologizes for getting in the way, it shows her walking up the stairs, and when it zooms out to get the full shot, she has yet to start walking up the stairs, and walks up the stairs.