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.
Revealing mistake: After Nuclear-Man kicks Superman in the air after putting the Statue of Liberty back, Superman does flips and part of him disappears.
Revealing mistake: When the subway train is supposedly running wild, watch the man to the left of Lois: He throws Lois's book violently away on the sly, as if the (fake) rumble inside the train had done the mess.
Continuity mistake: Nuclear Man sends the 2 police cars back and the one on the right skids and faces slightly to the right, but a second later, when they explode, both cars are perfectly aligned.
Continuity mistake: When Lacy shows the mock front page of the Daily Planet she holds it covering the red circle on the top right. A frame later she's holding it several inches down.
Continuity mistake: When Nuclear Man sends the police cars back, there's a man close to a mailbox on the left. There's a very brief cut before the cars explode but when the explosion takes place, the man has vanished.
Factual error: Superman and Nuclear Man fly around the world fighting at incredible speed. Everywhere they go, including places as widely spread as New York and China, it's daytime.
Visible crew/equipment: When Nuclear Man's laser beams send Superman flying away on the moon, the shadow of the crane moving him is visible.
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, the people next to her change completely between shots.
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.
Answer: There appears to be multiple reasons. They had creative differences, ultimately resulting in a poorly received movie. Kidder said Reeves, who co-wrote the story, had an inflated ego and clashed with Furie.
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Moreover, Mark Pillow, who played Nuclear Man, claimed that working with Reeve was quite intense.