After the electricity is neutralized, there's a motorcycle cop trying to start his motorcycle and the traffic light on the left side of the shot is lit.
When Klaatu visits Dr. Barnhardt in the evening, there is a small piece of paper pinned to one side of the blackboard, the left side. On the paper is seen (briefly) "Do Not Erase." A few moments later in the same scene, there is no paper pinned to the side of the blackboard. Again, in the same scene, the paper is back very briefly, but just as before, disappeared almost immediately.
During the montage shots of newspaper headlines during the hunt for Klaatu, a photograph shows him still standing on his ship holding out the viewing device which was shot out of his hand. During the earlier scene of his emergence from the saucer, he didn't produce the device until leaving the ramp and approaching the onlookers.
Bobby's room is clearly shown to be at the top of the staircase, and Helen's room is down the hall. However, in the shot where Helen is seen getting her coat from her room to go out with Tom to the movies, the top of the staircase can be clearly seen just outside of her door when it should be down the hall and much farther away from her room.
In Klaatu's hospital room, the light switch is on the wrong side of the door. Normally, the light switch would be on the same side as the door knob so the light could be turned on as you entered the room.
When the army is hunting for Klaatu the Colonel receives the license plate number and repeats it for broadcast as H0012. It should have been read as "Hotel" 0012" in accordance with the Military Alphabet Code.
Bobby's father's tombstone says Lt. Robert Benson was with the 45th Infantry Regiment of the 3rd Infantry Division and died at Anzio. The division fought at Anzio, but the 45th Infantry was not part of the 3rd Division.
When Gort puts Klaatu into the "life-regeneration" device, the blood stain on Klaatu's suit coat reveals no bullet hole present.
Klaatu gives Bobby several diamonds and tells him they're what his planet uses for money. However, the diamonds have the classic American Brilliant cut. This would be a very odd design for an alien civilization 250 million miles away.
Klaatu a being from a highly advanced, ethical race has no problem stealing a man's clothes and suitcase.
In some long shots of Gort walking, wrinkles in the legs of the costume are clearly visible. However, it could be that Gort is made from a very flexible kind of metal that is completely unknown on Earth.
Some suggested that it was impossible for Klaatu to have learned handwriting via radio; however, TV broadcast is also a form of radio wave.
Just after Klaatu is shot and is in the hospital, he is visited by a government official. Klaatu states he has traveled "Five of your Earth months" and "250 million of your Earth miles." 250 million miles would put his starting point between Mars and Jupiter, depending on when you measure. However, at the far end of the scale, Mars itself can get 250 million miles from Earth.
If temporarily stopping the world's electricity cut off all communications, there would be no way the meeting of military generals would know that hospitals and planes in flight weren't affected. On the other hand, it is possible planes were seen flying overhead, not to mention people were at the nearby hospitals who reported everything there was still operational.
General Cutler tells his board members during the power freeze that all communications in the world are out. Then an aide comes in and hands him a message from the President. The message was obviously sent via courier or received immediately before communications were interrupted.
At the beginning, H.V. Kaltenborn mentions the 'beautiful spring weather' in the Nation's capital, but we find out later that it is July (Klaatu sees a dry cleaner's receipt on his suit dated 7-18-1951). In spite of the spring and summer references, when Klaatu and Billy walk around Washington, many of the trees are still bare as they had yet to regain their leaves (Filming had started early in the year).
When Klaatu returns to the ship at night to contact his people, Gort is shown from the front after he knocks out the two soldiers. There is no cut-away as Gort turns to open the ship's ramp for Klaatu, and the zipper in the back of the rubber suit can plainly be seen as Gort walks towards the ship before he turns around to face front again.
When Gort goes to collect Klaatu, he passes Helen as he walks to the door of the central room in the spaceship and what looks like a large rip can be seen in the front of his otherwise solid-looking left leg.
When the military are trying to cut through the metal of the spaceship, a square of asbestos material taped to the ship can be seen, presumably to protect the wooden spaceship from the flame of the cutting torch.
In the shots from behind of Gort carrying first Helen, and later Klaatu, into the spaceship, the people being carried are clearly dummies - or rather, one dummy dressed first in Helen's and later Klaatu's clothing.
Klaatu spends a couple of days wandering round Washington DC and listening to the radio with the lodgers, then claims 'his patience is wearing thin.' Is he in a hurry?
In the close-up of the quick-triggered soldier atop the tank turret, he fires his pistol, but for the sharp-eyed, the wide-shot of the tank crew that follows, reveals the visible, but silent ,muzzle flash of both soldiers' side arms.
In one scene near the end, the sound of an airplane approaching can be heard, then there is a cut to another angle and the sound is absent.
Klaatu arranges to have the electromagnetic fields neutralized from 12.00 pm to 12:30 pm EST, yet it is clearly broad daylight in every country in which people are struggling with inoperative devices. In Asia and the Middle East, it should've been nightfall during this time frame.
When Helen and Klaatu are going to the professor's, the army states that their cab is heading northwest on Connecticut at Columbia Rd. The cab then passes under the Dupont Circle underpass on Connecticut AV. The underpass is south of Columbia Rd, not north. Also, if the professor lives near the State Department, they are going in the wrong direction. The State Department offices are south of Columbia Rd.
The fat man running crying, "They landed in the Mall," is wrong. The Mall is a long relatively narrow strip of land stretching from the Capitol Building and the Washington Monument. Yet Klaatu sets his saucer down in a square field with the South Face of the White House clearly visible at the edge of frame which means it is The Ellipse in President's Park. Furthermore, there are three baseball diamonds present: there are no baseball diamonds at the Mall. location.
As the spaceship races across the world, scenes of successive radio announcers (or people listening to them) are shown, apparently in order of their broadcasts. When the BBC announcer in England is shown delivering his broadcast, the clock behind him reads 8:33 (p.m.). The next shot shows radio commentator H.V. Kaltenborn doing his broadcast from Washington, D.C. The clock behind him reads 3:24. Since Washington is five hours behind London, and assuming the broadcasts were indeed shown in order, Kaltenborn's clock should have read no earlier than 3:33, and probably a minute or more later than that.
When the military is tracking Klaatu in the Yellow Cab, the radio call states that they a proceeding west on 15th Street. In Washington DC, numbered streets run North/South.
Not all vehicles would have been stopped by the power outage. Diesel engines, particularly at that time, don't need electricity to run. This would also explain why a boat can be seen moving in one shot.
When Klaatu boards his ship at night to arrange the electricity neutralization demonstration for the next day, he could simply have stayed on board where he was safe and emerged for the meeting the next evening. It made no sense for him to go back to the boarding house where, by his own admission, he was in danger.
Carpenter (Klaatu) is told that Bobby is quiet as a mouse, yet he pulls out a train set which makes a racket when he blows the whistle. In addition, he is a loud mouthed opinionated brat, hardly the type of child who is "quiet as a mouse."
Beth displays a complete lack of parental concern as she allows Klaatu, going under the name of Mr Carpenter, a strange man who simply shows up at the boarding house the night before, who shows no identification to anyone, to spend the day with Bobby, her underage son. She even tells Carpenter how nice it is for him to suggest it.
Since Klaatu has no money, only diamonds, he wouldn't be able to pay for his room in the boarding house, and it is doubtful that the owner would let a complete stranger move in without some kind of security deposit. She would be right since he flew back into space without paying any rent.
Klaatu tells Professor Barnhardt that he was in room 309 of Walter Reed Hospital, but the number on his hospital room door is 306.
The British radar man says it's moving at 4,000 mph. Then says it must be a Buzzbomb. As a British radar man, he'd know that the Buzzbomb only flew at 360 mph. 75% were shot down by the Royal Air Force.
When Bobby is doing his math homework, Klaatu looks over his shoulder and says: "All you have to remember is, first find the common denominator, then divide." Division of fractions doesn't require a common denominator. In fact, the original script says "subtract." (However, Klaatu's method could work. e.g. To divide 1/3 by 1/4 find the common denominator and re-express as 4/12 divided by 3/12. Then simply divide the 4 by 3 to get the answer -- 1 and 1/3. Ten out of ten for alien mathematics!)
A colonel orders his troops to block off all streets intersecting Connecticut Avenue "along a line from Wisconsin to the park." Connecticut Avenue and Wisconsin Avenue do not intersect.
Klaatu is from a highly advanced collection of races that have overcome all great social issues. One might assume that respect for the environment would be one of these, yet he litters, removing a laundry tag from his jacket and simply throwing it and the pin right on the sidewalk.