Continuity mistake: During the news broadcast, the TV announcer is evenly lit. You can see this in the side view of him, as well as on the TV monitor that's next to him. Then, just after he says "there is no reasonable cause for alarm," the microphone in front of him is suddenly casting a big shadow on him.
Continuity mistake: When Klatu goes to see the professor, there are several white notes tacked around the chalk board. In later shots some of the notes are gone. (00:39:25)
Continuity mistake: In the newspaper headlined 'Step up Hunt for Spaceman' the photo on the right shows Klatu and Gort on the hull of the ship. This shot is a still from the end of the film. (01:07:36)
Continuity mistake: Just after Klaatu gets shot, Gort is seen melting the clear glass/plastic/cement material he was encapsulated in. When Patricia Neal begins to approach the space ship, the camera shows Gort still melting the material with it almost down to his ankles by this time. The next shot shows Ms. Neal looking at him, but when the camera goes back to Gort, the cement he was encased in is suddenly back up around his waist. How did it get back up when moments before it was down around his ankles?
Continuity mistake: When the power goes out there is a scene where it shows all the cars not being able to work, although in the background there is a movie marquee with the lights on.
Continuity mistake: At the end when the spaceship is about to take off the people rush away from their chairs in fear of the launch. After the ship launches there are no chairs anywhere on the field, just the people.
Continuity mistake: After the power comes back on, a newspaper front page has Klaatu pictured with the president standing just outside the entrance to the saucer's dome, and Gort standing about five feet in front of Klaatu in the same place. But early in the film, Klaatu was on the ground with the president, and Gort never preceded Klaatu coming out of the saucer.