Continuity mistake: When the volcano erupts, the village's main tower keeps swapping form lit to unlit between shots.
Revealing mistake: While Superman and Nuclear Man fall off the building and fight on air, they pass by the same parts of the building over and over again.
Continuity mistake: When Superman is being buried in the moon, the flag pole behind disappears between shots.
Revealing mistake: When Superman and Lois jump off the window, Lois passes by the same window over and over again.
Factual error: At the beginning of the movie we're told that the Statue of Liberty is in Metropolis. However, the subway billboard states that it's in New York.
Continuity mistake: At the hotel room, Clark and Lois are watching the city skyline. From their POV the city is New York/Metropolis, but behind them is a glass door which reflects the real city, which is completely different.
Continuity mistake: When Superman flies to save the cosmonaut, the spaceship behind turns pink.
Continuity mistake: Before rampaging Metropolis, Nuclear Man elevates himself in Luthor's apartment and smoke comes out of his arms. For the rest of the movie this feature doesn't happen.
Continuity mistake: In the barn, the vest is folded in such a way that the sleeve-hole is hidden. However, in the close-up of Carl hiding the glass the hole is perfectly visible. Then it swaps to the previous angle.
Continuity mistake: Before Lacy enters the elevator at the hotel lobby an old man walks behind Clark, but suddenly disappears when the angle changes.
Continuity mistake: In the hotel room, the swimming pool's waters are flat, then when Superman arrives at the terrace behind they are very rippled, then a frame later it cuts to Lois greeting Super and they are totally flat again.
Continuity mistake: When the movie starts, the cosmonaut is standing up straight on the spaceship. A frame later he is laying horizontally plus he is suddenly attached to some sort of tube.
Continuity mistake: When the movie begins, the cosmonaut has some sort of big compass attached to his legs that disappears in the wide shots.
Revealing mistake: When Lacy floats in space, Superman saves her and heads towards Earth. When he comes back, the shot of the Earth behind is the same one from before, yet reversed.
Other mistake: When Lois and Superman fly together, the part where they fly over some horses is the very same shot as when they fly over the Golden Gate for the second time.
Other mistake: When Superman breaks his ice cage and makes it explode, a long black rubber band flies away. Where the heck did that come from?
Revealing mistake: When Nuclear Man melts the rifle, note that the SWAT member is holding the barrel and he slides his hand away slowly to make the barrel bend downwards. He was obviously holding it up. If he had burned himself he would've moved faster.
Factual error: Superman places the nuclear missiles in the giant disposable bag in space, throws them at the sun, and the bag burns up when it hits the sun. In reality, the bag would have burned up long before hitting the sun. (00:35:35)
Revealing mistake: Obvious doll replaces Nuclear Man when he falls into the chimney.
Continuity mistake: Lex is a bald man, except for this movie where he has very obviously natural hair.
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.