The graves on the headstones of the Wright children have the death date as 1986, but later when the girls are looking at the computer, Alex says the story is from 1996.
At the beginning of the film, 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 now overlooks the driveway at the front of the house.
When Matt visits Anna and her family for the first time to deliver some goods, he ties the boat to the dock with a rope. When Rachel sends him away, he leaves without untying the boat.
When Anna and Rachel are driving into town, the BMW is shown with a manual transmission. But later when Anna is trying to escape from the house in the BMW, it's now an automatic.
When Anna comes home and lies on her bed first time she touches plastic bird on the left side of bed. In the next shot when she turns around bird is on the right side.
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.
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."
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 Anna is at the party, Rachel tells her to take out the trash. When she opens the trash can, the sound of the lid opening can clearly be heard. So when she finally closed it, how come there wasn't any sound?
On the jacket cover of his book, David Strathairn's character is shown as Steven. It is listed as Stephen in the end credits.