Elena's slippers come and go when she is on land and in the water. One scene when escaping from the Nautilus, she clearly has the slippers on as she prepares to go in the water, the next cutaway has her in the water doing the breast stroke barefoot and no slippers in sight. Then she has the slippers when she is back on land.
On the surface of the water, you can see the masts of the sunken pirate ship extending above the surface. However, in the underwater shots the ship is fully submerged.
When the giant bird attacks Elena, she faints falling on her back. The bird places it's foot on her stomach. In the next scene she is laying on her stomach with the bird's foot on her back. Then in the next scene she is once again on her back with the bird's foot on her stomach.
When the pirate ship arrives, it is a three masted vessel with square sails on the first mast and fore and aft sails on the other two masts. The sunken pirate ship shows square sails on all three masts.
The "neck" of the balloon is shown blown out of position when first seen. It is then shown in place, too quickly to have been repaired.
A pirate ship would not fly a pirate flag whilst just cruising, but would unfurl one just before an attack to allow the target a chance to surrender. Whilst cruising, the pirate ship would fly a "false flag" such as a European country or US flag. Flying a pirate flag would just scare off potential targets or attract the attention of a patrolling warship.
In the first 22 minutes of the film during the narrative, a sulfur-crested cockatoo is to be seen. This is an indigenous bird found only in Australia.
Bamboo is not hollow, it is segmented with divided chambers which gives bamboo its light-weight strength. To make it into a pipe, the inside lateral walls that enclose the individual chambers have to be cut away.
African Americans in the US Colored Troops would not have been taken prisoner. They would have been sent to the Deep South as slaves, or simply shot on the spot.
Hot air balloons are held aloft by forcing heated air into the envelope of the balloon. The balloon in the movie had no means of providing hot air for lift off or staying afloat in the air.
While climbing the Volcano for the first time, as the camera pulls out, you can see the surrounding countryside - with a farm house and other homes in the background.
When Herbert is fighting the giant bird, we see the bird from a distance and it has very colorful feathers. When a closeup is shown of Herbert stabbing the bird in the neck, the feathers are white.
When two men are chopping down a tree with stone axes, large flat cuts in the tree trunk are seen which could only have been made by a saw that the men didn't have.
After they find the chest from the Nautilus, Mr. Spillit is shown shaving, but just after when they are discussing Captain Nemo, he still has a rather thick beard. Either the blade was very dull or he wasn't really shaving.
The smoke from the fire in the honeycomb would have asphyxiated Herbert and Elena before it grew hot enough to melt the wax.
When the soldiers are filling a drinking water barrel to take with them on their expedition, the captain is washing his hands directly upstream.
Captain Nemo sinks a pirate ship but none of the pirates swim ashore.
Elena is clearly wearing a (1960s) modern pair of yellow panties under her miniskirt. Also, no woman in the 1860s would have worn such a skirt which revealed her legs. It would be like being naked to the waist today.
The uniform worn by Michael Craig appears to be a dress uniform not worn by Union officers in the field. Unless he was taken prisoner at a Washington social event, it would be highly unlikely that an officer in the engineers would have been wearing such a uniform when captured. In addition, the epaulettes on his shoulders are those of an enlisted man; officer epaulettes have gold fringes hanging down. If he was captured in the field he would be wearing shoulder straps showing his rank, two bars.
Whilst in the cave, 50 minutes into the movie, Sgt. Pencroft sings part of the chorus of "fifteen men on the dead man's chest" from Treasure Island. The film opening credits state that the story is set in 1865, yet Treasure Island wasn't published until 1883, so Pencroft could not have known the song.
In the opening scenes of the film, we see cannons firing that have a system that allows the barrel to recoil to the rear on firing with the gun carriage remaining in place. This type of artillery did not exist during the Civil War.
When the pirate ship is boarded at the end of the movie, a Plimsoll line is visible on the side - this was not introduced until 1871.
After eating the giant crab, the five castaways begin climbing the volcano. In one long shot, a sixth shirtless person (apparently a crewmember) appears from behind a rock for a few seconds.
No explanation is given for how a hot air balloon could travel entirely across the United States and into Pacific Ocean in just a couple days, nor how Lady Mary and Elena became shipwrecked in the Pacific when they were on the other side of the world headed for Marseille in the Mediterranean.
During their underwater trek to the sunken ship the group passes both Greek and Egyptian ruins, neither of whose civilizations were anywhere near the Pacific Ocean.
After finding a sextant in the chest, the captain calculates their position as 36 degrees South, 153 degrees west. But a sextant can only measure latitude. Hence the famous longitude problem, eventually solved by accurate chronometer, which they lacked.
Captain Harding, his men, and the Ladies would not survive at sea since they have no food, water, maps or a compass on board the pirate vessel, nor any idea what direction to sail in,
After experiencing giant oysters, a giant crab, and a giant bird, Elena is looking at a wall filled with hexagonal shaped cells that are dripping honey and tells Herbert it can't be a honeycomb, it's too big.
As they hike up the rock mountain Mr. Spillett complains about his aging body, and envies the energy of youth, then only moments later is deftly scaling a 30 foot rock by rope.
It's amusing to hear Percy Herbert's refined and articulate English accent layered within the low-brow, southern accent of his character.
In explaining his plan to raise the sunken ship, Nemo says a ship floats "because it is filled with air". A ship, indeed any object floats when equilibrium is reached between the weight of the object and the weight of water displaced.
Captain Harding threatens Sergeant Bancroft with a 'burial at sea', despite the balloon being over land at the time.