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.
Continuity mistake: When Norman and Ethel are purchasing fuel for their boat, the total price on the fuel pump reads $19.00. As the boys watch Norman and Ethel leaving the dock the total price on the pump now reads $1.32. (Point of interest: as gasoline prices were increasing rapidly in the U.S. in the 1980's some of the older mechanical fuel pumps had a limit on the price per gallon so it was not uncommon for them to be set at half price and the totals had to be doubled ($38 instead of $19) until the pumps could be re-fitted).
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.