Continuity mistake: During the fight inside the spider, West wraps one of Loveless' men in a chain and kicks him out the door. You clearly see that the chain comes completely off the its roll and falls to the ground, presumably with the body. Yet later on in a wide shot of the spider you can see the body and chain still suspended from the spider. Then when it crashes, West uses the hanging body to save himself.
Continuity mistake: In the scene where the "Do not push" button is pushed for the first time, Jim West is launched on to the pool table and strapped down by two harnesses. However the lower harness shoots out between his legs, only strapping his right leg. In the next shot both legs are strapped down.
Continuity mistake: When Gordon and West are chasing Loveless in the train. They pass through a tunnel. On the other side of the tunnel, Loveless has elevated his train on crutches to hide as West and Gordon's train goes under Loveless' train. During the shot where you look through to the end of the tunnel there are no crutches showing where Loveless is hiding. In the very next scene you are looking back into the tunnel from the end and the crutches of Loveless' train are now there at the end of the tunnel. Then Loveless' train lowers back onto the track.
Continuity mistake: When Gordon and West are in the White House (after the real president Grant walks in), Gordon is shown removing his false stomach and tucking his shirt in. In one scene, the shirt is almost totally tucked in, in the next scene, the shirt is pulled back out of his pants. He then tucks the shirt back in again (for the second time).
Factual error: West and Gordon are outfitted with massively powerful magnetic collars. Somehow the two collars don't interact with each other at all until they try to remove them, when they prove able to attract each other from at least 20 meters. Gordon rationalizes this by saying that they accidentally reversed one magnet's polarity, but if this were so then the magnets should have originally opposed each other with the same force, instantly throwing each other apart as soon as they were brought within meters of each other. Nonsensical.
Other mistake: As the tarantula approaches the first small desert town (when Lovelace orders Grant to surrender the government to him), look at the townsfolk. Not one of them pays any attention to the noisy, giant metal spider as it lumbers towards the town. In fact, none of them react at all until the first building explodes. I guess giant spider robots were a common sight in the middle of the 19th century West.
Continuity mistake: When West and Gordon are being chased by Loveless in their trains they are riding on a straight (as we can see from their over the shoulder shots). Yet when Loveless fires the cannon at them, the shot shows the back of the train and a small hill being hit by the shot, the train is turning a corner as this happens. But when we see the O-T-S camera shot again, the train is back on a straight track. Where did the hill come from?
Continuity mistake: In the scene where Jim West is walking up to the White House we can see different shadows: the ones generated by the bodies of people in the street go one way, but we can clearly see that the white house dome has light coming from the same direction as the people's shadows are going. The White House's shadow goes in the right direction for the light.
Continuity mistake: When West is fighting the crew on the lower deck, he takes a chain and wraps it around one guy's neck. Then West pulls the chain so the bad guy is hanging, before West kicks him out of the spider. The other end of the chain is seen disappearing with the guy. But when the camera is switching back to West, the chain is seen on the right side, still moving upwards.
Factual error: Gordon shines a light through the back of the disembodied scientist's head to project his last visual image onto a screen. When the image on the screen is inverted Gordon remarks that "Refraction of the lenses causes the image to appear upside down". True; when an image passes through the lens at the front of the eyeball the lens inverts the image and it is upside-down when it hits the retina. However, Gordon is projecting the captured image on the retina back through the lens of the eyeball and onto the screen. When the image passes back out the lens would invert the upside-down image and return it to its proper orientation. The image should have been right-side up when it hit the screen.
Revealing mistake: When the saw blade thrower begins powering up after Jim West steps out of the magnetic boundary, the gears beneath the saw blade rise and begin to spin, which are meant to engage the saw blades' rotation, but in a single frame, these gears are shown instantaneously back in their resting places, as the saw spins on its own freely before being flung from the machine.