Continuity mistake: The SWAT people that are falling off the truck when it gets lifted disappear in the next angle where we see the pavement and a close up of the front of the truck.
Continuity mistake: When the window shops explode, a taxi cab door is widely open to show a Budweiser ad on it. In the following wide angle from above the door is closed.
Continuity mistake: Nuclear Man watches the Daily Planet paper with the "New publisher for your favourite paper" headline. The first shot shows the paper slightly wrinkled on the left and bent to the right. In the following angle it is straight and perfect.
Continuity mistake: In a split second, all of the snow on the mountain cut by Superman disappears.
Continuity mistake: When Superman saves the falling woman at the Great Wall, he flies in a 1 o'clock direction. A frame later he is positioned in a 12 0'clock.
Continuity mistake: When Superman stops the Soviet spaceship from turning around the solar panel is facing upwards, a second later the spaceship has changed position and the panel is sideways.
Continuity mistake: When Superman lands near the UN, Lois and Jimmy disappear from the crowd, only to appear half a second later, in the closer angle.
Continuity mistake: The Soviet missile has legible CCCP red letters, but in the following shot it's covered in snow, although the close-ups don't show any snow falling.
Continuity mistake: Outside the UN, the hot dog in the kid's hand suddenly disappears.
Continuity mistake: Superman is on the moon, about to cause the sun eclipse, and flies to the left, as if he were to exit the moon. In the next shot Superman is seen coming from the void towards the moon again.
Continuity mistake: When Lois is packing Superman's cape and Lacy says "I had nothing to do with it", there's no continuity with the passers-by seen behind. A kid in a red t-shirt, for example, appears from nowhere.
Continuity mistake: When Superman saves the girl from the Great Wall of China and starts rising, a woman with a white hat behind lifts her arm to say goodbye. In the following angle she is repeating the movement.
Continuity mistake: Before nuclear Man lands in the city, there's barely any smoke. A split second later, the close shot is completely filled with smoke. Then it disappears when we see the old lady with the yellow bag, and reappears when we go back to Nuclear Man.
Continuity mistake: While Superman is in awe reading the "Drop dead" headline, there's a group of journalists on the background and an empty chair on the right. When Clark stands up, a man appears seated on the chair.
Continuity mistake: After Nuclear Man lands on Metropolis a black man in a grey suit keeps moving from a white pole to a parking sign between each shot.
Continuity mistake: In order to stop the eruption, Superman flies towards a mountain. Watch the central part of it and you'll notice that the wide and close shots are of completely different mountains. The volcano also swaps from having smoke to exploding between frames.
Continuity mistake: Clark runs towards the subway train and stops. Then from a different angle he is several meters behind, running and stopping again.
Continuity mistake: The smoke coming out of the volcano changes direction in a split second after nuclear Man enters the crater.
Continuity mistake: In the quarry, Lenny parks his car to the very right of two wagons. Seconds later the car is in front of them.
Continuity mistake: When Superman is rebuilding the Great Wall all of the people near the hole vanish.
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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