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.
Continuity mistake: While Superman is taking the Statue of liberty back to its place the sky keeps changing between violet and blue, sunny and cloudy.
Continuity mistake: When Superman leaves Luthor in the quarry, he lands several feet to the right of the cop. In the close-up they are almost close to each other.
Continuity mistake: Shortage of extras in this film: Inside the UN hall, the arab man that greets Superman is the one that a second before was in the entrance, together with the woman with a pink handkerchief over her head.
Continuity mistake: When Superman lands Lex on the quarry, the standing position of the prisoner behind differs between shots.
Continuity mistake: After the two police cars are sent backwards, a third one that was parked behind, suddenly appears aligned with the other two.
Continuity mistake: When Lois is in the subway carriage crying for help, she places her right hand on the rail above. A frame later the hand is by her waist.
Continuity mistake: Before lava covers the Italian village, furniture under the two arches on the right, as well as a vase, cart wheel and table on the left disappear between shots.
Revealing mistake: When NuclearMan goes to destroy the Great Wall of China, there is a brief glimpse of him wearing support wires.
Continuity mistake: Nuclear Man arrives at Luthor's apt. and steps into a dark corner, where he loses his powers. From the opposite angle the place is filled with light, so Nuclear Man shouldn't have lost his strength.
Other mistake: The same front shot of Superman flying is used over and over and over again.
Continuity mistake: While Superman walks with the kid to the UN, the sky changes from sunny to cloudy in a nano-second.
Continuity mistake: In the gym, the amount of sport bags on the back differs between shots.
Revealing mistake: When Superman and Nuclearman are fighting for the first time in outer space, you can see them both standing on a small platform.
Revealing mistake: When Nuclear Man blows up the fire hydrant one can easily notice it's made of foam.
Visible crew/equipment: When Nuclear Man turns Luthor's assistant around on the air, supporting wires are visible.
Continuity mistake: While the lava flows on the Italian village, a wooden cart with hay on the left suddenly becomes a red car.
Continuity mistake: The debris on the bottom of the Statue of Liberty's base appears and disappears between shots.
Continuity mistake: Superman land on the middle-top of the missile, but a second later he is further down and on the side of it.
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.