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.
Factual error: In the scene where all four people get together after Jennifer Love Hewitt finds a body in her truck, Barry hits Ray with his right hand, which is in a cast. This would cause him a lot of pain, but despite using his injured arm more than once there's no sign of any discomfort.
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.
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 the Fisherman jumps out at the girl in the store when he is in the plastic bag, he misses and falls to the floor. If you look at the Fisherman when he is on the floor and waving his arms notice he isn't carrying his hook. However, in the next shot when he is standing up, the hook is in his hand again.
Plot hole: Barry gets killed violently at the pageant. So obviously there should be blood everywhere. When the police go up to the crime scene, they find nothing. There's no way the killer had time to clean up anything, let alone drag away Barry's body without someone seeing him. And blood starts dripping after the police leave the scene.