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)
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: 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.
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.
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.