Continuity mistake: When Superman throws Nuclear Man down the chimney, the sky is grey and cloudy, but a second later it's pitch black.
Continuity mistake: Lois visits Clark at his appartment and a wooden duck, with the head painted green and white and a black coloured body, appears on the table next to her.
Continuity mistake: Outside the UN, the crowd looks up to the sky and watch Superman come flying. When he lands, 2 seconds later, all the stunts on the first row are either gone or standing somewhere else. A kid on the right with dyed hair, for example, is now standing way behind the crowd.
Continuity mistake: On the Great Wall, a woman with a turquoise jacket stumbles and is about to fall off the edge. From a new angle two seconds later, she is three meters behind and stumbling towards the edge again.
Continuity mistake: After the volcano erupts, the boy standing to the right of the girl disappears between shots.
Continuity mistake: During the Great Wall of China scene, the sky changes from cloudy and grey in the shots involving people, to bright and sunny when the wall explodes.
Continuity mistake: When Lacy is about to enter the elevator, a light brown rug has appeared on the floor behind, which disappears in the next wide shots.
Continuity mistake: In the museum, while the guide is walking towards Superman's hair, people suddenly appear on the left taking pictures.
Continuity mistake: When Superman arrives at the UN, Jimmy Olsen stops in the middle of the street to take a picture of him. A shot later he's six feet behind a dark car.
Continuity mistake: While Superman flies with Lois, the sky keeps changing colour all the time.
Continuity mistake: When the statue of Liberty is about to crash on the street, a close shot is seen of jammed cars and hysterical people. When the shot changes to Superman flying to the rescue, the jam, the cops and the people have disappeared, and all is nice and quiet in Metropolis.
Continuity mistake: When the volcano erupts, there's suddenly a lot more snow on the crater.
Continuity mistake: When Lois opens the terrace doors in the hotel suite, the angle changes and the furniture on her right moves several meters to the left, appearing close to Lois.
Continuity mistake: In the gym, a man leaves a towel on a low wall. Watch the blue and grey stripes and you'll notice that its position is different between shots.
Continuity mistake: When Superman flies away from the tower holding the elevator, the first angle shows him leaving towards a 10 o'clock direction. A frame later the position of the cable shows him leaving to a 7 o'clock direction.
Continuity mistake: When Nuclear Man and Superman fall off the balcony and fight, the position of their bodies has no continuity in the wide and close shots.
Continuity mistake: In the quarry, the cop on the right's hat has two strings around the top, the upper one dissapears depending on the shot.
Continuity mistake: The UN building that Superman walks towards and the one he arrives at in a new shot look entirely different.
Continuity mistake: When Luthor introduces Nuclear Man to Superman, a towel on a chair changes from ironed and flat to a big bundle in less than a second.
Continuity mistake: When Nuclear Man is about to land on the Great Wall, the aerial view shows it's empty; a second later it's filled with people.
Answer: It was made on a very low budget. Golen - Golbus productions bought the rights to Superman. They were mostly known for B-Movies with not so big name stars. It was there attempt to play with the big studios. Plus at least 45 minutes of scenes were cut out, with major subplots.
In addition to budget cuts, they kept shortening the runtime, meaning scenes needed to be cut. The comic book adaptation has the uncut scenes and makes much more sense.