Visible crew/equipment: When Gort picks up Patricia Neal next to the flying saucer, you can see cables leading down to her body to keep her hoisted. (01:20:15)
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)
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)
Plot hole: Klaatu is shot and his body is lying in a prison cell. Gort leaves the flying saucer and walks all the way to the heavily guarded police station in downtown Washington DC. Gort is 8 feet tall and is a robot plus he uses his noisy death ray to create a hole in the police station to retrieve Klaatu's body and nobody noticed him? On top of that, he walks all the way back to the saucer unseen with a dead body. (01:22:36)
Factual error: In the very beginning of the movie broadcasters around the world are commenting on the mysterious spaceship. To lend a bit of calm and normalcy one broadcaster notes that despite what is happening many people are enjoying the fine spring weather in the Capital, to this end picnics and the wonderful cherry blossoms are plainly depicted. Shortly afterward when Klatu leaves the hospital, he is seen examining the cleaner's tag on the suit he "borrowed". This is where he adopted the name Carpenter. Clearly printed at the top of this tag is the date 7/18 1951, not spring by at least a few months.
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)
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: 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: 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 some shots, Gort the robot has air vents under the chin. In others there are no vents. (01:19:00)
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)
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)
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)
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: 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)
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)
Revealing mistake: When the spaceship arrives in Washington, many people look in the wrong direction. (00:04:20)
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)
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)
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.