In the coroner's office, Elizabeth's hair is shorter than it is in the rest of the movie, and she's wearing sneakers as opposed to her boots.
Elizabeth takes off her gloves when she first gets into Nick's truck near the beginning of the film. When the windshield shatters and Nick is struggling to control the truck, she is wearing the gloves again in a few very quick shots.
When Stevie hears the driftwood speak in her studio, it is 5:35. When it bursts into flames and she starts to put the fire out, it is 5:40. When she is done, it is 5:35 again.
When Nick & the group are at the church near the end of the film, Nick grabs Father Malone's whiskey bottle and smashes it. Whiskey is shown soaking one of the pews. A few minutes later, the same pew is shown again and the pew is perfectly dry. The amount of broken glass has changed as well.
When Sandy and Mrs. Williams are driving to see Father Malone, it is sunny and clear outside. As they drive up a hill, the day is suddenly cloudy. Then, when they arrive at the church it is sunny and clear again.
In a few scenes, Dan the local weatherman is tracking the fog bank on his weather radar, and giving reports. Weather radars have never been able to detect fog. Today's most powerful state-of-the-art NEXRAD radars are sensitive enough to detect bugs, birds, and smoke plumes, but still not fog.
When the fog cuts the telephone communication between KAB and the sheriff, the lines shown as being cut are actually the power lines, not the telephone cable.
A small (pull-start) power generator of the size that Adrienne Barbeau's character starts after the radio station/light house loses power would never be able to run a radio station's transmitter. Even a small regional station in this time period would have used vacuum tube radio equipment that consumed far more power than a small generator like the one that was shown could ever supply.
A cross made of solid gold, like the one that Blake and his men are seeking, would weigh several hundred pounds and would be far too heavy for an elderly priest to carry.
When the Weatherman calls the radio station and says that radar shows a trawler about 15 miles out and ID's it as "The Seagrass". Weather radars operate with the beam elevated one-half degree. This means that the farther away from the radar the higher the beams elevation. At the 15 miles stated the beam would be at 800 feet. Thus it would be very unlikely to show the ship. Also, weather radar cannot ID the name of a ship or it's see it's type.
Despite both the front windshield and the rear window being broken out on Tom's pickup truck, the wind doesn't cause the characters's hair to blow, regardless of the speed.
When Elizabeth & Nick are riding along in the truck after he first picks her up, we see the glass in the windows begin to smash inward. One can briefly see the object used to break the glass before each smash.
When one of the sailors peers out of the ship's window for a fog bank in the background, the superimposed fog appears over the edges of the window.
When Andy runs on the beach to find the gold coin, footprints from previous takes are visible.
When Nick and Elizabeth are in the truck when the glass breaks you can see Elizabeth's hand by Nick's shoulder wearing her gloves but she took them off when she got picked up.
when at the beginning of the movie, the bells strike midnight, according to Mr Machen it's the 21st of April. In the German translation Mr Machen says it's April 22nd. The translation is also otherwise inaccurate, e.g. he speaks of the sunken ship "her lungs filled with water and her eyes staring into the sea" when in the original it's the crew.
After making so much ado about how the fog breaks any glass that comes near it, the fog does not break any glass at Andy's house, through the town, at the radio station, or at the church (except physical ghosts punching through glass). However, this cost-saving plot hole is likely meant to be explained by the supernatural nature of the fog. Ghosts do not have to play by the rules.
As Father Malone flips through his ancestor's journal, you can momentarily see obscenities unrelated to the story written on the preceding page.
On board the Sea Grass Nick tells Elizabeth the story of how his father found a "Gold Doubloon, minted in Spain, 1867," on another ghost ship. However, the last Gold Doubloons minted in Spain were in 1849.
When Nick (at the bar) is on the phone with Stevie telling her about the SeaGrass, there is a cutaway to the lighthouse. To the left you can see a crew member motioning for more fog, and then more fog rolls by him.
The light used to illuminate the inside of Nick's truck is visible on the dashboard when the truck turns to go to the weather station.
Even though Stevie frantically announces over the radio for Sheriff Sims to call her because it's an emergency situation and the call is later cut off, he chooses to remain at the candlelight ceremony rather than drive out to the lighthouse to see if she's alright.
When the sheriff gets cut off from Stevie after ringing her at the radio station using the bar telephone as the message she had given out for him to contact her was said over the airwaves which was clearly heard, and the message was passed on to him, The next logical step would of been for Stevie to get back on the airwaves asking the sheriff or anyone who heard her message to contact the sheriff asking him to go to the weather station and check on Dan as she is concerned for his welfare.