Deliberate mistake: Before the school bus crashes, a brown car is seen coming. When the angle changes we still see a brown car but it's a totally different model.
Sacha
Deliberate mistake: Before Superman traps the black missile, its beam is really big and covers half of the lettering on the side. When Superman is behind, the fire is much smaller and the lettering is visible, allowing us to see Superman.
Sacha Deliberate mistake: At Lois' penthouse, before Superman says goodbye, the image is sharp and clear. Cuts to a close-up, and then check the next wide angle where he says goodnight and flies away: It's blurry and grainy because it's not live, but a screen with a previously shot scene to allow for the camera to turn left and, in a matter of seconds, no cuts at all, show Superman magically and quickly transformed into Clark Kent.
Sacha 
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.