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.
Continuity mistake: The Russian cosmonaut who says, "Sing at home, Sinatra.", is floating relaxed inside the spaceship, a frame later he is crouched fixing some bolts.
Continuity mistake: When Lenny and Lex do the genetic experiment, the stuff on the table comes much closer together between the first and second shot.
Continuity mistake: In the barn Carl looks ahead to see through the walls to check who's coming. His POV shows a cart wheel, but when the angle changes it's been replaced by 4 stables. The wheel is on the far right hand side.
Continuity mistake: When the volcano erupts, the village's main tower keeps swapping form lit to unlit between shots.
Continuity mistake: When Superman is being buried in the moon, the flag pole behind disappears between shots.
Continuity mistake: At the hotel room, Clark and Lois are watching the city skyline. From their POV the city is New York/Metropolis, but behind them is a glass door which reflects the real city, which is completely different.
Continuity mistake: When Superman flies to save the cosmonaut, the spaceship behind turns pink.
Continuity mistake: Before rampaging Metropolis, Nuclear Man elevates himself in Luthor's apartment and smoke comes out of his arms. For the rest of the movie this feature doesn't happen.
Continuity mistake: In the barn, the vest is folded in such a way that the sleeve-hole is hidden. However, in the close-up of Carl hiding the glass the hole is perfectly visible. Then it swaps to the previous angle.
Continuity mistake: Before Lacy enters the elevator at the hotel lobby an old man walks behind Clark, but suddenly disappears when the angle changes.
Continuity mistake: In the hotel room, the swimming pool's waters are flat, then when Superman arrives at the terrace behind they are very rippled, then a frame later it cuts to Lois greeting Super and they are totally flat again.
Continuity mistake: When the movie starts, the cosmonaut is standing up straight on the spaceship. A frame later he is laying horizontally plus he is suddenly attached to some sort of tube.
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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