Continuity mistake: Nuclear Man sends the 2 police cars back and the one on the right skids and faces slightly to the right, but a second later, when they explode, both cars are perfectly aligned.
Continuity mistake: When Lacy shows the mock front page of the Daily Planet she holds it covering the red circle on the top right. A frame later she's holding it several inches down.
Continuity mistake: When Nuclear Man sends the police cars back, there's a man close to a mailbox on the left. There's a very brief cut before the cars explode but when the explosion takes place, the man has vanished.
Continuity mistake: Nuclear Man lands on a street surrounded by skyscrapers. In the following shots where he destroys everything, the right side of the street is the same, but the left side is totally different: small houses, sunnier, signs on the walls.
Continuity mistake: While the volcano explodes close to the Italian village, a man jumps off a balcony but disappears just a frame later from the following wide shot of the place.
Continuity mistake: A couple seconds after Lenny insults Nuclear Man, when he is about to be lifted up, the place he stands in is different from where he was before.
Continuity mistake: In the hotel, right before Lacy enters the elevator, the hall of the hotel is filled with plants, couches and people. A second later, while Clark is on the trolley, the hall is a vast, empty place for the trolley to wander around.
Continuity mistake: When nuclear Man watches the newspaper on the table, the lighter is by the side of the phone. In the following shot it's in front of it.
Continuity mistake: When Lois gets in the train, the people next to her change completely between shots.
Continuity mistake: Before Superman rebuilds the Great Wall, he flies up with a woman in his arms. He flies past a stone arch and starts to descend. However, the following angle shows him yards behind descending right by the center of the arch.
Continuity mistake: Before Lex presses the red button to launch the missile, the phone next to it is hung-up. Then someone grabs it and the phone cable covers the control panel, but the immediate close-up of Lex's finger shows that the phone is hung-up, as in the first position.
Continuity mistake: The bundle that Superman makes to destroy the weapons changes to a tenth of its size in about a second.
Continuity mistake: By the colour of the new stones repaired, you can tell that the first gap that Superman fixes at the Great Wall is smaller in size and triangular shaped, instead of the original rectangular and huge gap made by Nuclear Man.
Continuity mistake: When Superman and Lois are flying together, the train below drives from right to left, but half a second later it is moving in the opposite direction.
Continuity mistake: After Lois and Clark read the headline with the "Drop dead" to kid, shots alternate between each of them and there's no sync in the graphics appearing on the computers behind each, while for the rest of the whole film all computers are in sync.
Continuity mistake: After the volcano erupts, a big black cloud covers the city, but a wider angle shows the sky clear.
Continuity mistake: After nuclear Man crumples the cab's hood, there's another taxi behind starting to drive slowly towards the damaged cab. A frame later, a new angle shows that taxi repeating the previous movements but really fast.
Continuity mistake: The SWAT van is lifted next to the curb, but a second later appears in the middle of the street surrounded by cars.
Continuity mistake: Luthor extends his arm with a dollar bill in hand for Nuclear Man to light. From the front his arm is fully extended. From behind it isn't.
Continuity mistake: When the SWAT van is turning around, the cars on the street are different than the ones in previous shots (try to find the destroyed taxi cabs that collided).
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.