Revealing mistake: In the scene where Ronnie cuts out a newspaper article about the UFO sightings, the night after Roy's first glimpse of the UFOs, two identical articles on Star Wars (1977) are on either side of the UFO article.
Revealing mistake: The morning after Ronnie and Roy's big fight, Roy starts cleaning up his clay "sculpture." Before he pulls its upper portion off, you can see the pre-made seam (which is poorly disguised by the clay) where the top is attached to the bottom. When he tears it off, the lower portion is perfectly smooth and flat on top (simulating Devil's Tower) where it was obviously pre-cut.
Revealing mistake: When the gigantic mother ship appears and sweeps over the rendezvous base at Devil's Tower, the camera cuts to look straight up at the advancing craft as it blocks out the starry night sky. Watch carefully to see some stars passing right through the leading edge of the mother ship, revealing a flaw in the traveling matte effect.