Other mistake: The same front shot of Superman flying is used over and over and over again.
Other mistake: When Clark Kent enters the subway, the set is the same as the rampage later in the film; only that the second time we see this set, the subway entrance has vanished.
Other mistake: Nuclear Man is shown blowing up the Great wall twice, but it's the same shot reversed.
Other mistake: In the rest of Superman movies, whenever he used his eyes to do some trick a beam came out. In this (really cheap) movie the beam is gone.
Other mistake: When the visitors are leaving the museum, the first man from the cue stares at the camera for a brief moment.
Other mistake: The subway carriage stops by the Statue of Liberty ad, however, the one reflected on the cabin's glass is a VW ad.
Other mistake: As Superman flies past the subway train, it's obvious that the train is sitting still, as the view of the tunnel shown it is stationary.
Other mistake: The part of the great wall of China that Superman fixes where bricks grow has nothing to do with the broken part from before, nor with the wall we see once he fixes it.
Other mistake: When Superman flies next to the subway carriage and Lois spots him and smiles, he is flying in the opposite direction.
Other mistake: Nuclear and Lacy are floating in space, not moving at all, but their hair moves strongly as if in an earthly breeze.
Other mistake: When Lois and Superman fly together, the part where they fly over some horses is the very same shot as when they fly over the Golden Gate for the second time.
Other mistake: When Superman breaks his ice cage and makes it explode, a long black rubber band flies away. Where the heck did that come from?
Other mistake: When Lenny has the prison guards locked in the car and is putting the top up, the steering wheel is spinning but the front wheels aren't moving. The amount of revolutions of the steering wheel is mechanically impossible also.
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.