Other mistake: Obviously, the spider needed a crew inside to manage its operation (feeding the boiler pressure going to keep it moving), but after West dispatches them individually, the spider seems to be able to run on its own when West and Gordon head back to Washington at the end of the film.
Continuity mistake: Gordon's train dodges Loveless' train-tank shot by going around a bend. In the shot right after the dodge, we should see the bend that they just passed, but all we get is more straight tracks.
Continuity mistake: When Arliss Loveless makes his appearance at a gathering, a giant bust of Abraham Lincoln is wheeled out. The head on this bust explodes and we see Loveless's head pop out in place of where Lincoln's was. When we see a close up of it, we see what was left of the bust around Loveless's head, yet instantly afterward in a wider shot, the remains of the bust disappear.
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. (01:02:20)Cody Erb
Continuity mistake: After the cornfield scene and the heroes fall into the mud river, their clothes and metal collars are seen to be covered in mud. In the next scene featuring them, their clothes are muddy but their metal ring collars are totally unmarked.
Factual error: In one scene Jim West is walking up to the White House. There is a cast-iron gate surrounding the White House, as there is today, but he gate was not added until the 1930's.
Factual error: The nitroglycerine would have exploded when McGrath's men threw it roughly onto the cart, leaving no risk of it exploding later when West makes his comment that this is not the way you transport nitro.