Visible crew/equipment: When the boy takes the boat out for a ride by himself and is riding it around in circles, you can clearly see the helicopter's shadow in the center of the circle.
Continuity mistake: Watch kid drive Norman's power boat. He is at the steering wheel, and appears at the left and right side of the boat in the same scene.
Continuity mistake: When the boy is dropped off at the cabin, Ethel's sweater is zipped down. It goes to a close-up of her and the zipper is up. In the next shot, the zipper is down again.
Visible crew/equipment: In the last scene, as the camera pulls away, the helicopter's shadow is visible on the surface of the lake.
Revealing mistake: When Norman and Billy crash the boat, the side panel rips off of it before they hit the rock.
Continuity mistake: When Norman is helping his wife uncover the furniture in the summer cabin, she hands him some sloppily rolled up sheets that drag the floor. The camera never leaves them, but for a few moments it zooms in and you don't see below their waists. When the camera zooms out again, the sheets in Norman's arms are all neatly rolled up.
Factual error: When Henry Fonda and Katherine Hepburn are filling the boat up with fuel early in the movie, when the gas pump is shown, Henry Fonda is looking at the pump. $19.00 is the total sale but 29+ gallons purchased, at .67+ cents per gallon... The total sale should be over $19.43, probably closer to $20 but I can't read the fractions of gallons or cents. (00:14:10)