Continuity mistake: In the plane, after Superman says that planes are safe, the man next to Lois raises his arm way up high, but a frame later, from an angle behind, he is barely raising the tip of his hand.
Continuity mistake: When Lois and Clark exit the building and she scolds him for leaving, the passers by don't match between one shot and the other.
Continuity mistake: A tube and oxygen mask suddenly appear over the seat in front of Lois after Superman enters the plane.
Continuity mistake: Clark's dog leaves a baseball on the ground and walks until being parallel to Clark's both legs. A frame later the dog is two meters behind, next to Clark's left knee.
Answer: There are two stories: a) the suit was made by Martha Kent from the blankets which came with Superman when he was a child and these blankets got superstrenght from the sun too. b) Superman generates an aura which protects him and his suit against such forces. Both stories can be true: either the suit itself is weakend by the kryptonite or Superman's aura was weakend by it. Currently it is said that the blanket-story is the correct one. Also, they don't actually cut it off him - it appears to open along a predefined line (coupled with someone exclaiming "got it!"), presumably the method Superman uses to get into the suit himself.
Bjoern_Buller