Continuity mistake: After flying with Lois, Superman walks away and is about to crash against the camera. He quickly dodges it by turning sideways. From the opposite angle he is walking straight.
Continuity mistake: When Nuclear Man arrives at the place for the first time Lenny is facing Luthor. A shot later he is facing backwards.
Continuity mistake: When Nuclear Man is created he extends his fingers. Shot changes to a wide angle and they are bent and he is extending them again.
Audio problem: After the satellite crashes against the cosmonaut, inside the spaceship a Russian cosmonaut starts to cry out things, but his mouth isn't moving.
Continuity mistake: When the satellite knocks down the cosmonaut, his helmet differs between the wide shots and the close-ups from smooth and with no details, to a real one, revealing the first one is a dummy.
Continuity mistake: The inside of the Russian spaceship differs between the outside shots as seen through the window, and the ones inside.
Revealing mistake: When Nuclear Man destroys Metropolis he sends beams to make things explode, but there's a moment when there are no beams seen and shops explode for no reason at all.
Continuity mistake: After Superman is pinned to the ground on the moon, he gets some dust on his suit and his hair is messed up, but in the following shots the dust is gone and his hair is perfectly combed back again.
Revealing mistake: During the Metropolis destruction scene, Nuclear Man sends a SWAT truck spinning around. When he lifts it up with his rays coming from his hands, you can see the truck is being lifted up by a visible white wire.
Revealing mistake: After Superman escapes the block of ice he was imprisoned in, watch the next shot (as he flies back towards Earth) closely... in this shot, it's clear that the effect was achieved by using a rather cheap-looking Superman "doll" composited into the shot, and not Christopher Reeve. It practically looks like a Superman "action-figure" you could buy at the store.
Revealing mistake: When Lex cuts the strand of hair, the bottom of the cabinet breaks before the ball drops.
Revealing mistake: When Clark and Lois are talking in Lacey's apartment, there appear to be bubbles and waves in the pool. However, they do not move.
Continuity mistake: From the aerial angle of the Russian parade, the trucks have already passed the tribune where the politicians are, but in the close-up they're many meters behind. (Laserdisc and international extended editions).
Continuity mistake: Superman flies towards the tornado, where the girl is, and "destroys" the tornado (or whatever it is he does), yet, for unknown reasons, Superman is suddenly seen miles away somewhere else, flying towards the tornado again and, what's even weirder: the girl is now floating on air. (Laserdisc and international extended editions).
Continuity mistake: When Superman leaves the farm, after the tornado, the basement doors swap from closed to opened between shots. (Laserdisc and international extended editions).
Continuity mistake: No soldiers are visible in the aerial angle of the Russian parade, but dozens in the immediate close-up. (Laserdisc and international extended editions).
Continuity mistake: When Superman blocks the Russian missile during the parade, its position keeps changing in every single shot. Sometimes it's close to the politicians, others away... This changes back and forth. (Laserdisc and international extended editions).
Continuity mistake: When Nuclear Man bombs the Russian truck during the parade, the politicians are standing apart, calmly. A frame later they are squeezed together horrified. This keeps changing half a dozen times. (Laserdisc and international extended editions).
Character mistake: When the little girl is about to be blown away by the tornado she is crying terrified, but when she is swallowed by the tornado, spinning around violently, she stays totally calm, even bored. (Laserdisc and international extended editions).
Continuity mistake: When Jenny is about to be sucked by the tornado, the windows are banging violently. The shot changes to a wide angle of her flying away and the windows are opened wide and still, despite the tornado blowing. (Laserdisc and international extended editions).
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 ★