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.
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 Kathy and George kiss on the bed, and in following shots, the mug on the desk to the viewers right of the bed constantly rotates itself, evident by the mug handle. (00:04:20)
Continuity mistake: Lisa's finger becomes bloody, as Jodie puts it into her bloody hole on her forehead. However, between shots, Lisa's finger becomes clean. (00:36:15)
Audio problem: When Chelsea is screaming "Liar! Liar! Liaar!!" to her mother, Kathy, in one shot we see Chelsea's mouth still saying "liar," but no audio is heard. (00:47:05)
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)
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.
Continuity mistake: When Kathy and George kiss on the bed, and in following shots, the blankets and bedsheets wrapped around George change. (00:04:20)
Continuity mistake: When George is laying in the bath and two arms grab him, the hands are all slit around the wrist, but in the next shot they're not. (00:42:25)
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)
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)
Continuity mistake: When the realtor shows the Lutz's the bedroom, and hears a noise, she turns her head to look. In this shot she is in the living room, not the bedroom. (00:09:05)
Continuity mistake: When the Lutz's look around the house, they enter a bedroom. Kathy walks up to a window and looks out. In the next shot George walks up to the same window - the whole window is in shot, but Kathy has gone. (00:08:50)
Audio problem: When the realtor suggests turning the basement into "A game room. Or a play room," her mouth says otherwise. (00:09:20)
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.
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: Mr. Defoe takes the gun down off his legs twice between shots. (00:01:20)
Continuity mistake: When Kathy reprimands Chelsea for standing on the edge of the boat, the balloon Chelsea holds swaps hands. (00:20:35)
Continuity mistake: There is a plate on the end of the dinner table, and egg which gets spilt on the dinner table which disappear between shots, as George serves the eggs. (00:05:40)
Answer: He sees the balloon, and instantly assumes his step-daughter is inside the boat house, and hence is in danger of slipping and drowning (or perhaps already in such a situation). He rushes straight out to get there as quickly as possible. Checking her room, or telling his wife would take up time, not to mention he is probably running on adrenaline.