Continuity mistake: When Clark sees Lex's building, there's no terrace anywhere, but when he arrives, a huge, wide terrace with an umbrella and plants around suddenly appears.
Continuity mistake: Lois visits sick Clark and opens the door wide. When the angle changes the opening is much narrower.
Continuity mistake: When Lacy acts sexy in her office, Clark is about to enter through her door, but when the angle changes he's 15 meters behind walking towards the door again.
Continuity mistake: After Nuclear brakes the cab's hood, the opening width of the trunk changes between one frame and another in between angles for no reason whatsoever.
Continuity mistake: When Clark is in Lacy's kitchen watching Lex's message, the billboard underneath changes between shots.
Continuity mistake: When Superman lands with Lois after flying around, he says "You don't know my name." Right then her bangs swaps from messy and uneven to evenly cut and brushed.
Continuity mistake: Superman's X-ray vision cooks the raw turkey and then the shot cuts to Superman's face, when it returns to the turkey is perfectly cook and now has a skinny leg on the right side.
Continuity mistake: When Superman lifts Lenny and Lex's car up in the air, Lenny's hand is inside the vehicle, but a frame later it's outside and further back.
Revealing mistake: During the subway sequence, all of the train stations are always the same Metropolis South.
Continuity mistake: Nuclear Man breaks through the Daily Planet ceilings but his hair doesn't ruffle, instead it looks much better than when he entered the building. Superman's hair does ruffle and get messy and dirty.
Continuity mistake: The Russian cosmonaut who says, "Sing at home, Sinatra.", is floating relaxed inside the spaceship, a frame later he is crouched fixing some bolts.
Continuity mistake: When Lenny and Lex do the genetic experiment, the stuff on the table comes much closer together between the first and second shot.
Continuity mistake: In the barn Carl looks ahead to see through the walls to check who's coming. His POV shows a cart wheel, but when the angle changes it's been replaced by 4 stables. The wheel is on the far right hand side.
Revealing mistake: When Nuclear Man destroys Metropolis, one of the walls between the window shop wiggles and the tiles flutter revealing it's a cardboard set. (01:10:55)
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.
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.