Revealing mistake: When the spaceship arrives in Washington, many people look in the wrong direction. (00:04:20)
Continuity mistake: As Drew Pearson is doing his news broadcast, the desk next to the microphone is clear. In the next frontal shot, two pencils have appeared next to the left of the microphone. (00:06:20)
Continuity mistake: As the spaceships runway platform emerges from the ship, there are hardly any trees behind the spacecraft; only a few behind the left side. In the next wideshot, there are plenty of trees behind the ship. (00:07:35)
Continuity mistake: When Gort emerges from the spaceship, we see all of the crowds run away in fear and panic. Yet in the very next shot the crowds are visible standing behind the ropes again. (00:10:05)
Revealing mistake: When Gort begins to walk down the saucer ramp, after Klaatu is shot, the crowd gets scared and runs away. I guess the director found the extras running too slow so he sped up the film. (00:10:30)
Factual error: When Michael Rennie is standing in the shadows at the boarding house, Mrs. Crockett turns the TV off with the channel selector knob, instead of the on/off/volume knob. (00:24:20)
Continuity mistake: When Klaatu posing as Mr. Carpenter is sitting with the others at the table, Helen says "Well after all, he was shot.." and Klaatu holds his newspaper up in his hands. In the next shot he holds it down on the table. (00:26:10)
Plot hole: In the scene at Arlington cemetery, Klaatu asks Bobby if it costs money to go to the movies. Bobby states he has $2 his mother gave him, which Klaatu trades for two diamonds. That being said, how in the world did they get from NW DC to Arlington in Virginia, and then on to the Lincoln Memorial at the Mall in DC? Much too far to walk, and Bobby made no mention of money he had before to take a bus or cab. (00:30:30)
Factual error: When Bobby and Klaatu are leaving the Lincoln Memorial, Klaatu asks Bobby where Professor Barnhardt lives. Bobby responds, "near where my Mom works, at the Department of Commerce." There is no residential neighborhood as depicted as the location of Barnhardt's house anywhere near the Department of Commerce. (00:32:05)
Continuity mistake: When Klaatu writes some notes on the blackboard, in order to help Professor Barnhardt, the handwriting of the words on the board changes between shots, most noticeably the 'T' on 'Try.' (00:35:00)
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: Klaatu introduces himself to the Professor for the first time by relaying some details of his stay at the hospital to verify who he is. Klaatu tells the Professor he was staying in room 309, but the early scenes at the hospital show him in room 306. (00:41:48)
Factual error: 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". (00:47:55)
Revealing mistake: After all of the elecrticity is neutralised, in a street shot you can see neon signs are still working; a huge Coca Cola one, for example. (00:59:30)
Revealing mistake: During the black-out, there's a shot when you see two cars moving in the background. One of them is slowing, which is normal if the engine stops, but the other is parked and then starts moving. (01:01:45)
Revealing mistake: The man from Mars is in an elevator with a woman when the power goes out. He tells her he cut the electricity and gas powered engines all over the world for a half an hour. They proceed to show clips of the power out in different parts of the world. When they get to Big Ben in London, you can see a motorboat cruising down the river in the background. If the boat were powered by wood or coal, there would be plume of smoke trailing it. If it were powered by oil (ie. diesel) electricity is required to pump the fuel into the ejectors. (01:02:05)
Continuity mistake: The taxi Helen gets into changes in one shot as it drives down the highway, evident by the writing on the side, then back again when it parks outside her house. (01:07:35)
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)
Revealing mistake: When Gort goes behind the wall to pick up Patricia Neal, the camera moves about an inch in the same shot, because they had to stop filming, go rig the wires up to Patricia, then start filming once again. (01:17:00)
Revealing mistake: When Gort picks up Patricia Neal, then heads back to the spacecraft, you can see a zipper on the front of Gort's costume, which is never there in any other shots. (01:17:05)
Answer: They said, how they handle their earth problems is no concern to them, it's only if they bring it to outer space and other worlds, they would take drastic measures to ensure that did not happen.