Continuity mistake: When Superman activates his laser vision to cut a mountain he moves his head from right to left, but from his POV the laser beam hasn't appeared yet, plus when it does its position doesn't match Superman's head.
Continuity mistake: Lois opens Clarks's door and stands in the shadows talking to him. The amount of light by the door swaps from gloomy to totally lit depending on the angle, with no order whatsoever.
Continuity mistake: Superman holds the tip of the mountain with his body straight, as if standing on air, yet a frame later his body is laying along as when he flies.
Continuity mistake: When Nuclear Man first shows up, a sculpture to his left appears and disappears between shots.
Continuity mistake: Amount of rock from the cut-out mountain standing out from crater differs between the first and second shot.
Continuity mistake: Lex greets Nuclear Man extending his hand parallel to the floor. A frame later the hand is tilted sideways, perpendicular to the floor.
Continuity mistake: A US flag appears and disappears from Lacy's table when she flirts with Clark.
Continuity mistake: When Superman is locked inside a block of ice, the first shots shows it's semi-transparent, but a frame later it's opaque.
Continuity mistake: Nuclear Man crashes through Lacy's ceiling to abduct her, producing a perfect squared hole, with lamps on each side. When he exits, the hole is smaller and with an irregular shape, plus the lamps are further apart than before.
Continuity mistake: When the Statue of Liberty plummets, the building behind changes style, and also swaps from bronze to concrete.
Continuity mistake: When Luthor steals the strand of hair, Lenny stands inches away from him. A frame later he is in a different position 3 meters away.
Continuity mistake: When Clark sees Lex's building, there's no terrace anywhere, but when he arrives, a huge, wide terrace with an umbrella and plants around suddenly appears.
Continuity mistake: Lois visits sick Clark and opens the door wide. When the angle changes the opening is much narrower.
Continuity mistake: When Lacy acts sexy in her office, Clark is about to enter through her door, but when the angle changes he's 15 meters behind walking towards the door again.
Continuity mistake: After Nuclear brakes the cab's hood, the opening width of the trunk changes between one frame and another in between angles for no reason whatsoever.
Continuity mistake: When Clark is in Lacy's kitchen watching Lex's message, the billboard underneath changes between shots.
Continuity mistake: When Superman lands with Lois after flying around, he says "You don't know my name." Right then her bangs swaps from messy and uneven to evenly cut and brushed.
Continuity mistake: Superman's X-ray vision cooks the raw turkey and then the shot cuts to Superman's face, when it returns to the turkey is perfectly cook and now has a skinny leg on the right side.
Continuity mistake: When Superman lifts Lenny and Lex's car up in the air, Lenny's hand is inside the vehicle, but a frame later it's outside and further back.
Continuity mistake: Nuclear Man breaks through the Daily Planet ceilings but his hair doesn't ruffle, instead it looks much better than when he entered the building. Superman's hair does ruffle and get messy and dirty.
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.