Deliberate mistake: The film supposedly took place in the Coastal Carolina region. This is a geographically flat area, called a coastal plain. In the beginning of the film, the kids were driving along ocean cliffs, which are not present in the Carolinas. Obviously, this was filmed in California. (00:01:15)
Factual error: Near the end, Julie James is crouching down by the front tip of the boat with a flare gun in her hand. Now, she is crouching so she is looking straight at the rest of the boat, instead of looking over the front of the boat. Then Ben Willis starts the boat with a big push and she falls back against the tip of the boat and the gun flies out of her hands. Now when you do that with a car, you normally fly backwards, but since Julie is turned around the other way, she should fall onto the piece of boat in front of her. (01:26:45)
Factual error: When JLH visits Missy at her home Missy says she went to Southport High, but in the July 4th celebrations the marching band has a banner which reads East Brunswick High School. A bit strange that a "small fishing village" would have two high schools for the simple fact that well the town probably couldn't afford it and it's pointless to have two when the village is so small. Factually, Southport does indeed have just one high school - South Brunswick High. See http://schooltree.org/NC-BRUNSWICK.html.
Continuity mistake: When Julie comes home and finds the letter, her lips are pale and she is not wearing lipstick. When the shot shifts focus from the letter to her face again, however, she is wearing purplish lipstick.