(at around 1h 30 mins) In the final battle, the fisherman is swinging the hook at Julie but hitting nothing but air. Yet the hook still makes the overused Hollywood "metal on metal" sound despite it not coming in contact with anything.
(at around 45 mins) When Julie and Helen are looking at David Egan's obituary, it mentions that he was 28 when he died. But when they go to see David's sister, she says that he was Class of '92 at the high school. So that would make him 23 when he graduated.
(at around 1h 30 mins) Near the end of the movie, in the final confrontation, Ray (Freddie Prinze Jr.) is knocked off the fishing trawler. He manages to snag the net, climb back on board the boat. He is completely dry.
After the car accident blood stains are visible on Barry's shirt. Barry dives into the bay to retrieve Helen's crown. When he emerges, the blood is mostly gone (presumably washed away). But seconds later when Barry walks up the dock, the prominent blood stains have re-appeared on his shirt.
Helen's father's store is called Shiver's. This is grammatically incorrect, as it should either be called Shivers as it is the name of the family, or Shivers' or Shivers's to indicate that it is owned by the family. It being called Shiver's implies that the family's last name is Shiver.
When the teens go to the dump the (supposedly) deceased man off the pier, he revives/regains consciousness and snatches Helen's crown. Despite knowing he's alive with strength to spare, they nonetheless drop him in the water and continually refer to him as "dead".
David Egan is listed to be in his twenties when he died, much younger than Ben Willis, the actual hit-and-run victim. This should have clued in the protagonists that David was not the person they ran over.
The goof item below may give away important plot points.
(at around 1h 12 mins) When Barry is murdered at the pageant, it's obviously a very brutal attack using a hook. Yet mere minutes later, the police can find no sign of blood in the balcony. This would not be possible.