Continuity mistake: Nuclear Man arrives at Luthor's apt. and steps into a dark corner, where he loses his powers. From the opposite angle the place is filled with light, so Nuclear Man shouldn't have lost his strength.
Other mistake: The same front shot of Superman flying is used over and over and over again.
Continuity mistake: While Superman walks with the kid to the UN, the sky changes from sunny to cloudy in a nano-second.
Continuity mistake: In the gym, the amount of sport bags on the back differs between shots.
Revealing mistake: When Superman and Nuclearman are fighting for the first time in outer space, you can see them both standing on a small platform.
Revealing mistake: When Nuclear Man blows up the fire hydrant one can easily notice it's made of foam.
Visible crew/equipment: When Nuclear Man turns Luthor's assistant around on the air, supporting wires are visible.
Continuity mistake: While the lava flows on the Italian village, a wooden cart with hay on the left suddenly becomes a red car.
Continuity mistake: The debris on the bottom of the Statue of Liberty's base appears and disappears between shots.
Continuity mistake: Superman land on the middle-top of the missile, but a second later he is further down and on the side of it.
Visible crew/equipment: In the quarry, when the cops' car flies up the air you can see the ramp used to lift it on the lower left side of the screen.
Continuity mistake: When Nuclear Man fires at the fire truck a short piece of hose comes out. A second later, in the following shot, the hose is 10 meters long and all over the place.
Other mistake: When Clark Kent enters the subway, the set is the same as the rampage later in the film; only that the second time we see this set, the subway entrance has vanished.
Continuity mistake: When Superman is giving his speech outside the Daily Planet, the background is a clear blue sky. In every other shot of Superman, the sky has gone all cloudy. (01:23:45)
Continuity mistake: When the satellite knocks the Russian cosmonaut, they are both dragged into space, but the immediate wide angle shows him alone in the void, no sign of the huge satellite.
Continuity mistake: While Superman walks to UN, the way Jimmy holds his camera changes in a split second.
Continuity mistake: When nuclear Man bursts the cab's hood, the radiator explodes and vanishes. A second later it's back on.
Continuity mistake: When Lois gets in the subway train it is packed with people, in the close shot they are gone.
Continuity mistake: When Superman and Nuclear Man fight on the balcony the high powered binoculars disappear.
Continuity mistake: After smashing his head, Superman sends Nuclear Man flying away. At first, Nuclear Man is conscious and with his head pointing upwards. A frame later he is unconscious and the head is laying down.
Answer: It was made on a very low budget. Golen - Golbus productions bought the rights to Superman. They were mostly known for B-Movies with not so big name stars. It was there attempt to play with the big studios. Plus at least 45 minutes of scenes were cut out, with major subplots.
In addition to budget cuts, they kept shortening the runtime, meaning scenes needed to be cut. The comic book adaptation has the uncut scenes and makes much more sense.