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.
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.