The Day the Earth Stood Still (1951)

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)

Reporter: I suppose you are just as scared as the rest of us.
Klaatu: In a different way, perhaps. I am fearful when I see people substituting fear for reason.

Trivia: In order to make Gort's body appear seamless, two different costumes were created, one that opened in the back for front shots and one that opened in the front for rear shots. Unfortunately, as someone has already pointed out on this site, it was sometimes rather difficult to co-ordinate which suit was needed for which shot and thus there are several times in the film where Gort's seams can be seen.