(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 59 mins) When the body is found in the trunk, its throat is clearly intact, contradicting its gruesome demise (at around 38 mins).
(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.
(at around 57 mins) The mirror when Helen is smashing it.
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.
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. Ben Willis's face was mangled and bloody so he couldn't be recognized, as a character mentioned.
The Fisherman who was right-hand dominant while using his hook throughout the film, had his right hand cut off by the lift on the fishing boat. During the shower scene at the end of the movie, the Fisherman left a note for Julie written in the same manner as before his amputation. Although unlikely, there is still the chance he is ambidextrous as he is never shown physically writing the notes.
(at around 1h 30 mins) When Julie is in the room filled with ice, it jiggles when she moves it (it was made of gelatin).
Barry punches different people more than once with his right arm, though it is supposed to be hurt and in a cast, and doesn't express any pain.
(at around 45 mins) David Egan's obituary says he died on Wednesday July 5th 1997, but in 1997, July 5th was a Saturday.
When Helen and Julie are researching David Egan on her computer, the browser shows the URL as "//http:cnty457.lib.org". It should of course be "http://cnty457.lib.org".
Sarah Michelle Gellar's hair extensions are plainly visible in closeups of her hair, before the killer cuts her hair.
(at around 56 mins) When Helen goes to sleep the bow on her lamp shade is facing the end of the bed but when she wakes up it is facing the wall behind her.
When Barry is walking around the gym after he takes a shower, you can see an outline of this shorts under the towel.
When the killer is pursuing Helen and Helen's trying to get into the store, Helen obviously should have ran off. Not waited there as the killer walked towards her.
The killer walks towards Helen rather than runs. He should know that walking towards her will likely allow her to easily escape.
(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.
(18:55) While Barry, Helen and Ray are talking Julie into dumping the body instead of going to the police Max driving at them distracts them. When Barry first heard the tires of Max's truck he looks over his right shoulder and headlights are visible from Max's car but then Max comes from Barry's left side after his headlights were just seen coming from his right side.
(at around 1h 17 mins) When Helen is running from the killer, when she approaches the front door of the store, the cameraman is reflected in the glass.
Although the action takes place on the east coast of the USA in North Carolina, at the very beginning of the movie we see the sun setting over the ocean.
(at around 14 mins) As the body hit the automobile, the kids were driving on mountainous terrain along the coast. There are no mountains anywhere in the North Carolina shore area.
There's no explanation given as to how the killer would know where to send the letters or know where Barry worked or where Helen lived or knew she would be in the parade etc.
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".
The origin of the photograph displaying Barry's wrecked car is never explained in regard to who took it, or where it was taken.
There's no explanation given as to how the killer would have survived rather than drowned.
Melissa shouts "Get out of my house!" to Julie, when both of them are standing outside Melissa's house.