Revealing mistake: When Superman saves the people that Nuclear Man throws up into the air, as they lower you can see the support wires holding them up.
Continuity mistake: After Superman straightens the US flag, a brief shot shows he is on a plain ground, but when he is knocked down by Nuclear Man, a big mount appears in front of him.
Continuity mistake: When Lacy is in outer space grabbed by Nuclear Man, she slides down dramatically and grabs on his wrist. Half a second later, the sliding effect is gone and she is now floating relaxed, grabbing the upper part of his arm.
Continuity mistake: Mr. Warfield asks the little boy to repeat "I wish Superman had said yes" and places his hand on the kid's shoulder. The shot changes inmediately to a close-up and the arm and hand are gone.
Continuity mistake: While the people in the village are watching the lava flow down the streets, the first shot shows them to the right of a wooden door. A frame later they are positioned several feet further, close to a big green plant.
Revealing mistake: When the net full of nuclear missiles is thrown into the sun, the footage of the explosion is exactly the same as the explosion of the single rocket with the DNA of Nuclear Man attached.
Continuity mistake: When Nuclear Man buries Superman, there's a huge rock near the hole. When Superman comes out, the rock has disappeared.
Continuity mistake: When in the subway, Superman is standing by a 6 foot poster of the Statue of Liberty. When the shot changes to a flying Superman, he is a third of his size and the poster is huge.
Continuity mistake: Behind where Nuclear Man lands there's no school bus on the wide shot, which is a matte painting, but a frame later one appears and stays there for the rest of the scene.
Continuity mistake: Clark enters the barn to fiddle with his green crystal. When he exits, a huge amount of stuff from his childhood has magically appeared around the place.
Revealing mistake: Watch the moon fight between the two supermen. In one scene, Superman charges nuclearman and you can see the backdrops flutter if your monitor brightness is high.
Plot hole: For some strange reason, when Nuclear Man passes away in space, he remains floating, however Lacy, who is grabbing his hand, falls downwards.
Continuity mistake: The close shot of the Great Wall exploding shows an edgy architecture, with straight lines. The wide shots show a curvy construction.
Continuity mistake: After Superman lets go off the elevator on the surface of the moon, the cables fall down with the elevator, but in the close-up the cables are up and fall down again.
Continuity mistake: A couple walks behind the hot dog stand in the same direction twice in less than half a second.
Continuity mistake: In the barn, Superman puts a glowing crystal inside a pocket. A frame later it disappears.
Continuity mistake: Luthor holds a sealed champagne bottle with both hands on the neck of it. He doesn't move them, nor his shoulders, nor do we hear any sound revealing that the bottle's being opened. However, when Luthor leaves, the bottle's seal appears opened and 20 centimeters away when the scene ends.
Continuity mistake: When the truck is carrying the missile, the officers parading in front of it pass by the same place twice.
Continuity mistake: Before Superman enters the UN hall, the amount of people by the staircase changes between shots.
Continuity mistake: During the volcano eruption scene, an old man with a black hat on the right appears and disappears depending if the shot is a close-up or a wide shot. This even happens when the shot changes follow one another immediately.
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.