Continuity mistake: When Billy is holding logs still for George to chop, the second log he puts on the stump is stripped of any bark. But when George swings down to chop it, the log is suddenly covered in bark. (00:50:45)
Continuity mistake: When George is awakened by the dog barking, George walks down the hall with his shoes tied. In the next shot, as he exits toward the opened door, his shoes are untied. The following shot shows George walking across the deck with his shoes tied once again. (00:54:55 - 00:55:45)
Continuity mistake: When George rips the phone from the wall, a long cord is attached to the back of it, but in the next shot only short frayed wires are on the back of the phone. (01:08:05)
Continuity mistake: After Kathy gets her hair caught in the boat's rotor, she runs back to Chelsea, through a downpour. But when George appears, he is dry, despite having to have had to run through the downpour as well. (01:08:40)
Continuity mistake: After Kathy falls into the water and gets her hair caught in the boat's rotor, she climbs out of the water and onto the deck. Between shots her hair becomes much drier and more stringy, and her face is dry. (01:09:20)
Continuity mistake: When the children and Kathy push the large wooden desk in front of the door, to barricade George in the basement, the items on the desk (cloth, doll, books,) repeatedly change position, disappear, reappear etc. (01:10:45)
Continuity mistake: At the end of the film, George falls off the roof and lands on the mud, getting his face and shirt extremely filthy. However a bit later, when the family escapes on the speedboat, he awakes and his face and shirt are now exceptionally clean. Admittedly the rain could have washed off his face, but not cleaned his shirt to that extent. (01:13:05 - 01:17:15)
Other mistake: Towards the film end, when George climbs up stairs with the gun to kill others, he walks into the passage and shuts the door. A mirror is there in the passage, but there's no reflection of George in it. (01:15:15)
Revealing mistake: When Chelsea is on the roof of the house and Kathy joins her, Kathy's hair is blowing in the wind, while Chelsea's stays perfectly still, the water in the lake is also perfectly still. Obviously the director had the actors do the scenes separately until we see them both together in which case both characters hair is blowing around. (01:27:40 - 02:00:00)
Character mistake: When Lisa is taken to the hospital, she tells Kathy that she saw the ghost of Jodie DeFeo. The name of both of the DeFeo's daughters wasn't Jodie. Their names were Dawn and Allison. Jodie was the name of a demonic pig that the Lutz's daughter Missy had befriended.
Suggested correction: While this is true in the original novel, the writers chose to change it to the DeFeo's daughter's name in the movie. Creative license doesn't count as a mistake.
Visible crew/equipment: When Kathy is bringing in the groceries and she asks George where Chelsea is, they run outside to look for her. As the scene goes along it pans past the truck and you can see the reflection of the camera in the truck.
Deliberate mistake: In the beginning, the police chief says "The six members of the family were found last night, in bed, shot and killed." As he says this, look to the left of the screen. In the background, the real Amityville house (dark brown with white windows and trim) is shown instead of the house used in the rest of the film. That segment was from actual news footage of the murders from 1974; apparently, the director didn't want to recreate it.
Continuity mistake: When the main character picks up the shotgun (at the beginning of his rampage) he cocks it. He then cocks it again as he goes up the stairs, but he hasn't fired it yet. This would have ejected an unused round, but nothing comes out.
Revealing mistake: Ryan Reynolds is wearing red-eye contacts. You can see the whites of his eyes around the red circle of the contacts in several scenes - most noticeably, right after the home movie he's watching down in the basement melts.
Suggested correction: Not a mistake. Ryan Reynolds actually had bloodshot eyes from being awake for nearly two days of filming. He stated it in an interview for the movie.
Continuity mistake: Towards the end of the movie after George falls, and the rest of the family is climbing down, we see (from top to bottom of ladder) Kathy, Chelsea and then the eldest kid, with the youngest boy still reaching out on the roof. Yet, in the next shot, the eldest kid and Kathy are on the ground, and Chelsea is on top of the roof again, jumping into Kathy's arms.
Factual error: There is a poster for Whitesnake on Billy's wall. This movie takes place in 1975. Whitesnake didn't form until three years later.
Factual error: There is an Alice Cooper poster on the wall of the boy's bedroom. The logo on the poster was not used before the 2000's.
Continuity mistake: When George kills the dog in the boat house, he leaves the axe behind. Later when he goes after Kathy, it's back stuck in a block of wood by the house.
Other mistake: The type of pan George uses to make eggs is far more modern a design than when the movie is supposed to take place in the late 70's.
Revealing mistake: Towards the end, when the family is getting away from the house in their boat, you can see that the motor is a covered camera. It is too high out of the water to be a real motor and when the shot is not from behind the boat you can easily see the "motor" moving left and right while the boat is still.
