Continuity mistake: Nuclear Man lands on a street surrounded by skyscrapers. In the following shots where he destroys everything, the right side of the street is the same, but the left side is totally different: small houses, sunnier, signs on the walls.
Continuity mistake: While the volcano explodes close to the Italian village, a man jumps off a balcony but disappears just a frame later from the following wide shot of the place.
Continuity mistake: A couple seconds after Lenny insults Nuclear Man, when he is about to be lifted up, the place he stands in is different from where he was before.
Audio problem: When Lex is giving instructions to launch the missile, a young man puts on his earphones and talks to Lex, but his mouth doesn't move.
Continuity mistake: In the hotel, right before Lacy enters the elevator, the hall of the hotel is filled with plants, couches and people. A second later, while Clark is on the trolley, the hall is a vast, empty place for the trolley to wander around.
Continuity mistake: When nuclear Man watches the newspaper on the table, the lighter is by the side of the phone. In the following shot it's in front of it.
Continuity mistake: When Lois gets in the train, an old man with a grey gabardine enters first and is seen behind her. Then the shot changes and the man has changed to a baldie middle aged guy.
Continuity mistake: Before Superman rebuilds the Great Wall, he flies up with a woman in his arms. He flies past a stone arch and starts to descend. However, the following angle shows him yards behind descending right by the center of the arch.
Continuity mistake: Before Lex presses the red button to launch the missile, the phone next to it is hung-up. Then someone grabs it and the phone cable covers the control panel, but the immediate close-up of Lex's finger shows that the phone is hung-up, as in the first position.
Continuity mistake: The bundle that Superman makes to destroy the weapons changes to a tenth of its size in about a second.
Revealing mistake: Although Lois and another passenger move violently to show that the train is running wild, the rest of the passengers and the straps to hold one self remain perfectly still.
Other mistake: When the visitors are leaving the museum, the first man from the cue stares at the camera for a brief moment.
Visible crew/equipment: When Nuclear Man enters the building after being told he would be taken to Lacy, he flys upward through different floors. As he's doing this you can see support wires attached.
Continuity mistake: By the colour of the new stones repaired, you can tell that the first gap that Superman fixes at the Great Wall is smaller in size and triangular shaped, instead of the original rectangular and huge gap made by Nuclear Man.
Continuity mistake: When Superman and Lois are flying together, the train below drives from right to left, but half a second later it is moving in the opposite direction.
Continuity mistake: After Lois and Clark read the headline with the "Drop dead" to kid, shots alternate between each of them and there's no sync in the graphics appearing on the computers behind each, while for the rest of the whole film all computers are in sync.
Continuity mistake: After the volcano erupts, a big black cloud covers the city, but a wider angle shows the sky clear.
Continuity mistake: After nuclear Man crumples the cab's hood, there's another taxi behind starting to drive slowly towards the damaged cab. A frame later, a new angle shows that taxi repeating the previous movements but really fast.
Audio problem: When Nuclear Man knocks out Superman on the moon we hear some growling, but their mouths aren't moving.
Continuity mistake: The SWAT van is lifted next to the curb, but a second later appears in the middle of the street surrounded by cars.
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.