Goofs

When Rob breaks into Becky's house it's clearly night when he enters and leaves but lighter outside when he is in the study and photographs the letter to Eliza. Also Rob's black gloves suddenly go missing with no explanation when he takes the photograph (but are present preceding and following this).

When Rob breaks into Becky's house and holds up the envelope with Becky's mother's name on the front, it shows it as 'Liza Snyder', but when the mother goes to work the next day and calls the school, she identifies herself as 'Eliza Snyder'.

When Becky's mom leaves her office to go to the school in response to a fake phone call, she walks toward her car, clicks the remote to unlock it, and gets in. Rob is already in the car, even though the car has obviously been locked until the moment she gets to the car.

The movie takes place in Southern California. However, in the aerial scene immediately prior to the birthday party (around 1:13 into the movie), the night scene shows Ocean Drive (near 9th street) in the South Beach area of Miami Beach, Florida. several thousand miles from Southern California. In the top right of the scene, the illuminated cargo cranes at the Port of Miami are visible.

Rob's elaborate scheme to kidnap Eliza - by impersonating a school vice principal - seems completely unnecessary. When he breaks into Becky's house, Eliza comes downstairs to investigate alone and the knife wielding Rob potentially has her at his mercy. Also this is an isolated location at night, so confronting Eliza and Becky there makes far more sense than doing so in daylight at the high school where many other people are around.