Continuity mistake: As Elizabeth is being driven home in Nick's pick up truck, all the glass suddenly shatters. It is shortly before 10pm (clock at her mom's house). But despite the fact that Nick and Elizabeth are busy all the next day, the truck is over 30 years old, and the story takes place in a small, island community, all the truck's glass is back when it's seen the next afternoon - both the front & rear windshields, door glass and vent-windows, and it's all just as dirty as it was before the breakage.
Continuity mistake: When the Seagrass is attacked, the two girls are thrown through the cabin's rear windows, landing on the outside deck. When the boat's owner finds it, a camera circles the Seagrass from a distance and the rear windows are intact. When he and Elizabeth board the boat, the windows are again broken.
Factual error: It's been established, that the burning of the lepers ship happened in 1871, yet when one of the city's "founders" pulls his gun on Blake, it's an 1873 Colt Single Action Army, a gun that didn't exist 'til two years later.
Continuity mistake: When Nick crashes his truck into a boat and trailer, Elizabeth is shown being tossed around inside the truck in slow motion as the truck settles to a stop. But in the scene showing the aftermath of the crash, she's regaining consciousness on the ground a good fifty feet or so to the right of the truck.
Continuity mistake: In the scene where the paramedics are removing the bodies from the Seagrass it shows them putting a sheet over the one girl and guy that were killed and moving them into the vehicle, which would have left one other body. but in the next shot from above there are two bodies laying on the dock with sheets over them.
Continuity mistake: When the two men and two women are on the Seagrass, the women are in the cabin when the Fog first strikes. The windows fog up and the blonde woman draws a scale on the window. The drawing differs between her drawing it and the close up. Notice the lines on her drawing extend beyond the base lines of the scale the close up lines do not.
Factual error: When Stevie's car is struck by a delivery truck, the collision is head-on for both cars. The force of the impact should have pushed her car rearward (even if slightly to either side). But when the camera cuts to show the car's passenger side from well below, the car is turning onto it's side, rolling toward the camera as if a forklift lifted the other side of the car, tossing it off the road. The car had no rearward motion at all, despite the head-on collision.
Continuity mistake: Stevie's radio station is shown located in a lighthouse, yet the night shot early in the film shows an entirely different lighthouse than is seen in daylight mid-way through the film. The later version's top, red portion isn't nearly as tall, and it has at least a third of its glass replaced with solid panels, while the earlier one is taller and all glass.