Continuity mistake: There is a "No Riders" sign that keeps appearing and disappearing on the window of the jeep, from one scene to another, fron shot to shot, throughout the entire film.
Revealing mistake: The alien is starting to mess with all sorts of mechanical and electronic devices. One of the first ill-effects shown is a large industrial cable reel that is dangling in midair when the mechanical arm lifting it is paralyzed. Problem is, you can see the operator has his hand on the controls and is stopping the crane himself using the lever, nothing alien about that. (00:14:50)
Revealing mistake: Toward the end of the film, when Claire Anderson enters the cave where the monster is waiting, she passes a distinctive rock formation. In a later scene, where she is going further into the cave, she again passes the same rock formation.