The Nautilus attacks the ammunition ship from port side. Yet in a subsequent underwater shot, the Nautilus is seen breaching through the hull from starboard side.
When Ned sees the giant squid starting to sink and taking Nemo with it, he draws his knife from his belt and dives in to save Nemo. Arriving at Nemo's position, the knife is back in his belt and needs to be drawn again.
The Nautilus violently shakes after it rams the war chemicals ship causing it to explode, but the bottles on the shelves in the viewing room haven't moved at all.
When they wait outside Roa Pandi for the sailing ship to leave harbor, the Nautilus is seen floating closely to the island. When Cpt. Nemo orders Full speed ahead in a subsequent shot, the Nautilus is shown at open sea.
After Ned and Conseil return to the Nautilus they do not secure the rowboat or close its cover, yet Nemo does not see to this before going after the warship.
When the giant squid appears, it is swimming toward the Nautilus with its tentacles first. While squid can swim in both directions, they normally move mantle first with tentacles trailing due to much better movement through the water and their gill systems adapted to movement in this direction, particularly if they are trying to swim at a high rate of speed. Also, if the squid was moving with the tentacles in front, they would trail toward the back, not stay rigidly in front of it, like a person's arms stretched out.
The film opens on what purports to be a copy of Jules Verne's original novel. The book is turned to Chapter One, which is titled "Alarming Rumours!" The actual title to Chapter One in Verne's novel is "A Shifting Reef."
The shark that goes after Ned and Conseil is a nurse shark. This species is actually quite harmless and shy of divers.
Captain Nemo electrifies the hull of the Nautilus to drive off the squid and the natives. Electricity needs a complete circuit to work. Since the Nautilus is submerged when the squid attacks the sea water would have shorted out the electricity and prevented it from harming the squid. Likewise, since the natives are standing on the hull and are not grounded they would also have not been harmed.
When Captain Nemo is hosting his guests for dinner, he informs them that what they think are fruit are really sea cucumbers. The sea cucumber is an animal, not a vegetable.
When the Nautilus gets back underway after sinking from the New Guinea reef, Nemo states, "There are limits beyond which man and his puny efforts cannot survive, we exceeded them by 5,000 feet." Although not for another 100 years was any undersea craft built for such depth capability, and then only small research submersibles, the conceit of the film is that Nemo invented many advanced technologies far ahead of his time.
The note that Ned Land puts in the bottle, while containing numbers representing both latitude and longitude, neglects to say whether the latitude is North or South. The note is however not useless: the unnamed state could have sent warships to both the North and South locations.
When Ned Land drinks the jar of alcohol, it is obviously nothing but clear liquid in the jar. With Captain Nemo's genius, it is quite possible that this is a form of vodka or even moonshine whiskey (both are totally clear), made from materials found in the sea. There are a variety of alcoholic drinks which are clear.
When Prof. Aronnax and Capt. Nemo return to the Nautilus in the dinghy, the horizon is slightly curved, revealing the scene was actually filmed in a big basin.
When an attacked ship explodes, the men of the Nautilus are thrown AWAY from the explosion. If there was a real explosion, the Nautilus would be blown away from the explosion, and the untethered men would be thrown TOWARDS the explosion.
When Nautilus is going through the underwater tunnel, there is some shots from inside the wheelhouse. One can see cliffs passing by directly under the windows. The wheelhouse is situated on top of the submarines hull, so there is no way that the cliffs can be so close to the windows.
When Captain Nemo and the Professor return from the mines, the sailor is rowing the skiff in one direction but its actual movement is in the opposite direction.
During the undersea fishing sequence, there is a shot of a sea turtle that is stretched out of shape because it is non-anamorphic footage that was stretched out by the wide screen lens.
When Ned enters the galley, he lifts the metal lid off of a steaming pot without burning himself.
The "New Guinea natives" do not look like actual natives in New Guinea. The actors and costumes are African.
The title appears without a comma between the first two zeros: "20000 Leagues Under the Sea".
When the natives are coming down the stairs of the Nautilus, the lead native is wearing a wedding band.
Ned plays a tenor guitar (four strings as opposed to six), which was not invented until the 1920s.
DVD subtitle gaffe: When Ned Land and Conseil are looking at the wall map in Nemo's cabin, Land asks Conseil to hand him the "calipers", written "calendars".
When Ned Land is supposedly accompanying himself on the guitar, the chord positions of his left hand do not match the music.
After the Nautilus attacks the cargo ship leaving RorapandI, Ned, Conseil and Aronax watch the aftermath out of the mid-ships viewport of the submarine, but in the upper right of the shot, the silhouette of the bows of the Nautilus is visible. There is no way someone on the Nautilus could see the ship attacking the quarry at the same time.
When the giant squid first appears, you can see a human hand lifting it up in the bottom right corner. Widescreen version only.
Shortly after first entering the Nautilus, Ned, Professor Arronax and Conseil watch a submarine funeral service taking place on the ocean floor. Considering the Nautilus is at the surface at the time yet is only ten feet or so above the ocean floor, this makes the water unrealistically shallow for the middle of the ocean.
The professor's accent sounds nothing like a French accent.
In the dining room scene, Ned objects violently to his food after Nemo describes for him what he's actually eating. Considering that everything on the table came from the sea Ned's reaction would not be consistent with a hard-bitten harpooner, who would have eaten all manner of strange food, both marine and terrestrial. His hammy reaction was no doubt a bit of pandering to the kids in the audience who would have identified with him thoroughly.