Revealing mistake: All the footage of the missiles being launched is of a poor quality and grainy, which is inconsistent with the good quality of the rest of the shots, revealing that it's stock footage.
Revealing mistake: Superman has just saved Jimmy at the dam and is holding him in the air. Jimmy's fringe is standing up thanks to some hair fixer to achieve the effect of wind blowing (though it's a green screen effect). When Superman sets him on the floor his hair remains up giving away the trick, until the angle changes and it looks all combed and nice.
Revealing mistake: When Superman takes Otis and Luthor to the prison wires holding them are visible.
Revealing mistake: When Superman leaves the prison, wires holding him are visible.
Revealing mistake: When Superman lands the burglar, wires holding him are visible for a brief moment.
Revealing mistake: Under Grand Central, the train that passes by Otis is the same one (5048) that kills the cop.
Revealing mistake: In Grand Central Station, the same extras appear in every scene, no matter where in time or place occurs.
Revealing mistake: After Superman saves the cat, he takes off and wires holding him are visible.
Revealing mistake: Before the Hoover Dam breaks, the places where all the cracks and water will later appear are easily traced on the concrete, covered in plaster and painted. Either that, or the real Hoover Dam has burst many many times and always through the same spots.
Answer: When Krypton exploded, the resulting debris was chemically altered through nuclear fusion, converting it into kryptonite. Bits traveled through space, some eventually ending up on Earth, where it is now lethal to anyone who was from that planet.
raywest ★
Pieces of Krypton that exploded in the Red Sun were made radioactive, and the Red Sun is one of the weaknesses of the Kryptonians.
I thought the Red Sun was poisonous to Kryptonians and caused the remnants of the planet Krypton to become radioactive and also absorb some of the solar energy from the Red Sun. I was under that impression, maybe I'm mistaken.