After the dramatic face-off between Esther and Kate in the icy pond, Esther advances out from under the water, begging Kate to save her and is holding her knife behind her back. However, when Kate kicks Esther back into the pond, the knife disappears from view.
When Kate climbs out of the ice hole in the pond, she is wet and her hair is wet when she picks up Max. But when there is a close up of her kissing Max, Kate's hair is dry. Her hair is wet again in the next distance shot.
Early in the film, at one point, the family returns home and it is snowing lightly. The windshield wipers stop and there is a brief overhead shot of Kate near the car with a lot of snow collected on the windshield. Not enough time had passed to allow that much snow to collect on the car.
When Kate is shopping in the grocery store with Esther and Max , Sister Abigail calls Kate on her cell phone. As the camera cuts between Esther and Kate, the produce display changes without the cart moving.
When Kate and John first meet Esther and speak to her while she paints lion cubs, her painting progresses a lot quicker than it should for the amount of time that passes on screen. Also in some of the close up painting shots, it's obvious from the fingernails that someone else's hand was used for those shots.
When Kate is talking to Grandma Barbara in the kitchen about her drinking problem, she starts to eat a bit of the cake on a plate by the sink with a fork. However, every time you see the cake as the scene progresses, the amount of it left and the positioning of it is different.
When Esther is introduced to her new class, she is holding a Nelson Spelling book. These books are not used in the United States, where the film is set; they are, however, used in Canada, where filming took place.
When Esther tries to kill Danny in the hospital, the ECG (green curve) and oxygen (blue curve) alarm only go off after she puts the oxygen sensor back on Danny's finger. However, the ECG is not monitored using the oxygen sensor, so the ECG alarm should have gone off as soon as she started to suffocate Danny.
After Kate enters "antisocial personality disorder" into an Internet search on her computer, as reflected in her glasses lenses, the screen immediately directs to a synopsis page clearly labeled with a different personality disorder, Borderline Personality Disorder. The two disorders are not synonymous.
(at around 1h 55 mins) In the credits the word "Russia" in Russian from Esther's papers is spelled wrong. The first letter should look like the Latin character "P," not "R" as was shown in the film. Cyrill P is Latin R. CPPP = S.R.R.R.
The Smith & Wesson m-36 holds 5 rounds, as clearly shown when she removes it from the safe and opens the cylinder. Esther fires five rounds when chasing Kate through the house. When Max picks it up by the pond, she fires a "magical" sixth round.
When Daniel is playing Guitar Hero on his Xbox 360 console, his controller power light is not on, indicating the controller is not on/or plugged in to the console. Also, when the game is shown on the television Daniel is playing on the 'Expert' difficulty and appears to be doing quite well, however looking at his fingers, Daniel is only pushing the first three buttons on the guitar (easy mode) and not all five buttons as Expert mode requires.
When Esther catches John and Kate having sex in the kitchen, the reflection of the couple in the glass shows John holding Kate against the counter with one arm. In the same shot Johns arm is not holding her down. Also, as John turns his head away from the glass, his face can still be seen in his reflection. The reflection appears to have been added in post production.