Revealing mistake: Obvious doll replaces Nuclear Man when he falls into the chimney.
Revealing mistake: When Nuclear Man flies through the ceiling for the first time, the first glass he crashes into is noticeably different than the others surrounding it, revealing it's a prop.
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.