In the beginning, Anna looks out her bedroom window to see the lake at the back of the house. In the penultimate scene, her father stands in her bedroom looking out the same window, which is now facing the driveway at the front of the house.
Near the end when Rachel has brought Anna back from the sheriff's office and flings her on her bed, her arms land so they are laying above her head. In the next shot when Rachel has her back turned, Anna's arms are at her sides even though she hasn't moved. When Rachel turns around again, Anna's arms are again over her head.
Rachel dumps the large cut of meat onto a board with four sharp prongs after she takes it out of the oven; these boards are designed to skewer the meat so that it will be held still while carving and not move or roll around. Despite this, when Anna bumps into the table, the meat simply rolls off the board - which it could not have done on such a board.
When Alex and Anna are looking online for information about the Wright children murders, they come across an article which ran in a local paper about the event. The story mentions that the children were apparently "heavilly" (incorrectly spelled) drugged when they were killed.
Pay close attention to the bags of groceries and the box of supplies that Matt delivers to the dock. Although the bags are apparently full and the box contains some heavy looking items, it's obvious by the way the girls easily lift and toss them around that they indeed contain nothing but empty boxes, bottles, etc.
When the sisters read the Internet article about the Mildred Kemp murders, they show the article having the phrase "had became" in it. It is, however, read aloud with the proper phrasing, "had become."
The goof items below may give away important plot points.
On the medical aide application that Anna looks through to find out more about Rachel's true identity, the application indicates that it should be written clearly in BLACK ink. The form is filled out in blue ink.
Three different dates of birth are presented for Rachel Summers throughout the scene where Anna and Alex try to find out about her true identity. On her social security card, her date of birth is listed as November 4th 1955, while on her birth date on her application for the job caring for Anna's mother is written down as April 14th 1975. On her driver's license, it lists her birth date as July 22nd 1976. Even if her social security card is a fake and the identification of the "real" Rachel Summers, the date of birth on her application and license should have matched.
In the final scene, Mildred Kemp is holding the triple-stranded pearl necklace. In a mental ward for the criminally insane, she would not be permitted to have a necklace for the same reason the patients aren't allowed to have shoelaces: they could strangle or injure themselves or others with them. That's why all of the mental patients are walking around in socks.
In the beginning of the movie, Anna's mother is in the boathouse, ringing the bell that is tied around her wrist. The boathouse blows up with her in it. However, later in the movie, Anna finds the same bell in perfect condition, and the ribbon around it still intact.