Continuity mistake: When Superman is walking with the kid towards the UN, the sun changes between shots.
Revealing mistake: Right after Lex, disguised as a soldier, orders the launching of the missile, there's a shot of the truck carrying it. The driver is a cheap mannequin that looks very much like a scarecrow.
Continuity mistake: When Superman starts his speech at the UN, in the wide angle his arms are extended, but a frame later they're next to his body.
Continuity mistake: When the satellite hits the spaceship, the shots from inside show the spaceship turning around clockwise, yet in the outside shots it is moving randomly: up and down, clockwise and anti clockwise.
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.
Revealing mistake: Due to the blue-screen effect, part of Superman's glasses disappear when he is free falling with Lois.
Visible crew/equipment: Boom mic reflected on the hood of the cop car's, during the city attack.
Plot hole: Nuclear Man sends laser beams pointing his arms and destroys everything around, yet when inside the Daily Planet, he sends beams to the elevator but absolutely nothing happens.
Visible crew/equipment: When Lenny lets the guards in you can see filming equipment reflected in the rear-view mirror of Lenny's car.
Continuity mistake: When Superman flies towards a mountain, the lava on the volcano behind him disappears.
Revealing mistake: Watch the cars and the grey truck during the LexTv scene. Whenever Luthor appears on screen the same cars are driving on the street.
Continuity mistake: In class, Jeremy has his body bent over his desk. When the angle changes he is sitting properly on the chair.
Continuity mistake: At the Great Wall of China, Nuclear Man points his hands forward and explodes the wall in front. As of then, there's no continuity between the position of his hands and the location of the places exploding.
Continuity mistake: When Lenny leads his car with the cops in, the car is facing a very short road ending in a small mount of rocks. The following wider angle shows a very wide open space with grass and a huge cliff.
Revealing mistake: It is way too obvious that when Luthor holds the dollar bill, his arm and hand have been replaced by a robotic prop. His arm manages to move without involving all the bones connected to the shoulder and the back.
Revealing mistake: While Superman carries the Statue, there's a close shot of Nuclear Man showing his fingernails. The movie behind is playing the wrong way, and gives the sensation that Nuclear Man is flying backwards.
Audio problem: When Superman is wounded on the neck, and when the italian villagers are saved, the same male falsetto voice is used to add a (crappy and out-of-sync) "Oooh!" and "Aaaah!".
Continuity mistake: When Superman enters the UN hall, a back shot shows the people in the upper bleacher clapping, but a frame later, the front shot shows them sitting still.
Revealing mistake: The close shot of people panicking when the Statue is about to crash against the pavement shows an old woman with a red wool hat and a white scarf. Many scenes later, when Nuclear Man lands on Metropolis, the same lady is seen, wearing the same clothes.
Continuity mistake: While the crowd follows Superman to the UN, the man with the balloons is first seen far away from him, but half a second later he is very close.
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.
raywest ★
Moreover, Mark Pillow, who played Nuclear Man, claimed that working with Reeve was quite intense.