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)
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: 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)
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)
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)
Continuity mistake: When the Lutz's get their first house call, the clothes in the suitcase, on the end of the bed, change between shots. (00:14:20)
Continuity mistake: When George locks the boat house door, you can see inside the boat house through the cracks in the door, but there is no boat visible inside even though there was a boat there in the previous shots. (00:21:10)
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 George cooks eggs, the egg spills over the frying pan. In the next shot it doesn't. (00:05:30)
Continuity mistake: When Kathy and George are in the Italian restaurant, between shots George picks his wine glass up twice. (00:30:25)
Continuity mistake: When Harry, the dog, scratches at the wall, a large slab of concrete laying on the floor, by the wall, vanishes. (00:26:05)
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.