When Frank is on top of the boat looking down waiting for the gangsters to come out, a view of him from above shows a shiny plated gun in his left hand. The rest of the scene shows him with a black plated gun in his right hand. However, he had picked up "Toots'" gun after shooting him. That gun was shiny.
When Johnny Rocco racks the slide of his semi-automatic pistol and then holds it to Frank McCloud's belly and repeatedly pulls the trigger, a loud click is heard every time he pulls the trigger. However, a single-action semi-auto pistol such as the one shown can only release the cocked firing pin when the slide is racked, a shot is fired and a new round chambered, or the hammer is pulled back manually. None of those occurred after the first trigger pull.
Mr. Temple describes fairly accurately the hurricane that struck the Florida Keys on September 2, 1935. However, his description of the death toll ("800 people washed out to sea") is about double that of the official toll of 408.
At the beginning of the film, as the hurricane approaches, all doors and windows are Shuttered. Interior lighting is thus kerosene lamps. Later, the shutters are no where to be seen, and lots of sunlight stream into and light rooms.
At the beginning of the film, when the police car pulls up next to the bus, the bus is empty (no people). But, a few seconds later when the bus stops and the policemen go in, the bus is full of people.
Frank, Nora, Mr. Temple and the gangsters are in the hotel's big hall (which is downstairs) when the phone rings, Curly takes it, and then they draw their guns. Then comes a brief dialogue between Rocco and Curly, but when we return to the party, we see them in a little living room upstairs, without having knowledge of them being carried there.(Perhaps intentionally made by the producer).
When Rocco is getting a shave he tells Rocco 'That's enough of that" but still has a lot of shaving cream on the side of his face and a big spot missed. When he looks in the mirror he is completely clean shaven.
Many palm trees are correctly shown bending before the winds. However, one rigid, unbent palm topples over (1:06:29). Palm trees survive in hurricane-prone areas precisely because they are flexible enough to bend instead of being toppled.
Describing the summer heat in the Keys, Mr. Temple says that it will cool down in November: "The thermometer will drop to 100 (F, about 38C)", implying that summer temperatures are consistently above 100. This is incorrect. The climate is humid, but the adjacent waters moderate the temperature. Average summer high is 87F (31C). According to the Keys official tourism website, "The hottest it's ever been in Key West is 97º F (36º C), and that was way back in 1880."
Mr. Temple says of the oncoming storm that it is unusual to have one this late in the season, and that they usually happen early in the summer. In fact, the Atlantic hurricane season peaks near the end of summer, around September 10. Hurricanes in early summer (June-July) are much less frequent.
The goof items below may give away important plot points.
Errors in geography
Near the end the sheriff tells Mr. Temple, Nora and Gaye that the state police have picked up Ziggy and his henchmen as they were about to cross the border into Georgia. It's about 485 miles from the Keys to the Florida-Georgia border. As Ziggy and his men drove away not long before Rocco and his men left on the boat, and Frank had shot Rocco and his men only about 12 miles off shore, it would not have been possible for Ziggy and his men to reach the border of Georgia by car in so short a time.
Once Frank is aboard the boat with the gangsters, Temple tries to use the hotel phone, but the line is down from the storm. But after Frank has dispatched all of the gangsters, he phones the hotel from the boat and is able to get through. Not nearly enough time has passed for the phone to have been repaired so quickly, especially in such a remote area.