Continuity mistake: Lois exits Perry's office and Clark enters to talk about salary. His hat swaps from facing up to down between shots.
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Continuity mistake: When Clark is about to jump in front of the train, there's a crossing sign shaped as an X. Depending on the shot the front upper left part is either bent and about to break, straight, or missing.
Sacha Continuity mistake: When the Kent's stop the truck and see the cradle, the sky swaps from sunny and clear to heavily cloudy in a second.
Sacha Continuity mistake: When Kent's father pulls over his car, he holds the steering wheel with his right hand on the top. From the side, barely a frame before, he has his left hand on the side.
Sacha Continuity mistake: The way the bags are positioned in the Kent's pickup differs between the side and back angles.
Sacha Continuity mistake: In Lex Luthor's lair, the bottles to the left of the TV broadcasting the news disappear in the following shot.
Sacha Continuity mistake: When hanging from the bridge, the damage on the school bus' hood is different depending on the shot.
Sacha Continuity mistake: When Ms. Teschmaker is sunbathing, the trolley by her side is close to her thigh. In the wider angle is by her ankle.
Sacha Continuity mistake: When Jimmy runs towards Hoover Dam his hair is long. A fraction of a second later it's very short.
Sacha Continuity mistake: After Jor-el talks to Von-Dah he places his hands on a man's chest. In the following shot they're on his shoulders.
Sacha Continuity mistake: In the gas station, the "closed" sign falls down but appears up in the next shot (you'll find it in the right side of the screen).
Sacha Continuity mistake: Perry White shows newspapers with day shots of Superman, but he made his appearance at night.
Sacha Continuity mistake: When the burglar falls from the building, his flashlight is sometimes swinging, but other times it's flat against his body revealing the camera was tilted.
Sacha Continuity mistake: When Lois dies, Superman's face is covered in dirt. When he takes off and flies, it's clean.
Sacha Continuity mistake: When the gas station falls down, watch the right side: A green car with an open hood suddenly gets replaced by a red car.
Sacha Continuity mistake: When the school bus smashes the Golden Gate fence, the aerial shot from the back shows the fence complete, the shot from the front shows it smashed.
Sacha Continuity mistake: When Lois dies, the first time Superman sees her the left side of her face has sand and rocks around her cheek. When the angle changes they are at shoulder level.
Sacha Continuity mistake: When Superman avoids the train from derailing, the wide shot shows his cape fluttering but the close one not.
Sacha Continuity mistake: When Superman moves the school bus away from the void, the wide shot shows a different fence and wreckage, with a long piece of fence missing. The close shot shows a missing space that is slightly bigger than the bus.
Sacha Continuity mistake: When Superman saves the wrecked plane, from the outside shots the window has droplets of water, but from the copilot's POV the glass is clean.
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Answer: Superman has chosen to use his powers on Earth for the good of mankind. It would seem that lying to them would be an unneccessary thing to do, so it appears to be a personal choice. Interesting trivia: the closest he comes to lying is when he tells Lex Luthor to get Zod and company into the molecule chamber in the second film. Listen closely, he only says that it is a molecule chamber that takes away their powers, he never actually says that being inside will make them lose them, only that he wants Luthor to get them to go into it.
JazetopherThat may be the closest "Superman" comes to lying, but Clark Kent does appear to lie. After the mugging scene in the first movie, Clark tells Lois the contents of her purse, and when asked how he knew (obviously from his X-ray vision), he says "wild guess" which was a lie.
jimbaNot only that but, he spent his whole life pretending to be ordinary so, in fact, he spent his whole life living a lie.