Continuity mistake: The little motor boat that G. Peck uses appears to have lost its windshield from the time he is driving it to when it is tied up.
Factual error: Cape Fear the town, the river and the cape, are all in North Carolina. The civil-rights-type lawyer tells Bowden that a committee sitting in the state capital will see that he never tries another case in "this state." That committee turns out to be in Atlanta. The Atlanta trip is a vital part of the plot - the word "Atlanta" occurs about a dozen times in the script.
Max Cady: I got somethin' planned for your wife and kid that they ain't nevah gonna forget. They ain't nevah gonna forget it... and neither will you, Counselor! Nevah.