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).
Revealing mistake: All of the shots of Nuclear Man flying, or him turning around to check for Superman are the same ones repeated over and over again.
Revealing mistake: During the tornado scene, the wind makes the windows open and close violently, but the cowboy hat on the porch stays in place barely moving. (Laserdisc and international extended editions).
Character mistake: The Daily Planet newspaper headline "New publisher for your favourite paper" has "favourite" is spelled the British English way. Since Metropolis is in the United States it should be spelled "favorite."
Continuity mistake: The floating pliers behind the cosmonaut keep appearing and disappearing between shots.
Continuity mistake: The cosmonaut is fixing a satellite dish with his left arm but a frame later he's working with his right arm instead.
Continuity mistake: On the moon, Nuclear Man grabs a rock and dirt covers him totally, especially his armpit becomes black. A frame later, he's spotless.
Continuity mistake: When the volcano's lava floods the village, there's a wide shot of the mountains and the volcano and you can see that the lava is still running down the volcano, not near the village.
Revealing mistake: When the volcano erupts and lava floods the village. The red truck on the left is a very obvious scale model prop: The engine is missing and the hood is attached to the grilles.
Revealing mistake: When the fire truck explodes, note a tube behind it going straight to the truck. Probably an FX device to effect the smoke.
Continuity mistake: When the firemen are about to grab the hose and it explodes, note that the other hose inside the truck on the right side has changed positions between shots.
Continuity mistake: Nuclear Man walks towards the newspaper where Lucy's picture is, for a brief moment there's a light smoke coming from behind him as if something was burning (it has nothing to do with the smoke Nuclear produces when flying) which is later gone (according to SupermanIV.com a deleted scene involved Nuclear Man waking up, "he sees and picks up the newspaper with Lacy, which then catches on fire in his hand.").
Continuity mistake: Before Nuclear Man see Lacy's picture in the newspaper, he rises from the floor and a wide angle reveals that the table is nowhere around, yet a couple shots later it simply appears half a meter away from him.
Revealing mistake: When dying and balding Clark grabs the green glass, the border of his wig noticeably protrudes all around his face.
Continuity mistake: When Lois visits sick Clark she stands by the door, fully lit, a frame later, when the angle changes, the whole place is in the shadows.
Revealing mistake: When Superman discovers that Nuclear Man is stealing the Statue of Liberty, he turns around and you can see the wrinkles on his temple from the wig he was using.
Continuity mistake: When Superman first meets Mr Warfield, the orange pencil on the table changes positions between shots.
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.