Continuity mistake: When the group go out to inspect the house damaged by the monoliths, the dirt around the rocks is white while the rest is dark. The professor squats down and picks up white dirt in his left hand and dark dirt in his right hand, then calls the geologist. In a close up, he opens his hands and the dirt colors have changed hands - the shot has been reversed.
Continuity mistake: When the geologist parks in front of his office, the back of the station wagon is a good six feet left of the door. When the newspaper man walks across the street to visit, the back of the car is now even with the door. In a later scene when the other geologist arrives, the back of the car is once again about six feet from the door.
Other mistake: About seven minutes before the end, the geologist and company and a Highway Patrol car go to the edge of town to get a better view. As the Highway Patrol car skids to a stop look on the road and there is already a longer skid mark. I'm guessing that in the first take the car skidded too far.