Continuity mistake: In the scene between Martha and Clark near the end of the episode, the cut on Martha's forehead switches from above her right eyebrow to her left, and then back again. (00:37:40)
Continuity mistake: When Zod/Lex pierces Lana's hand, he pierces it at the same level as Lana's head. But in the camera angle immediately following, Lana's head slide downs the wall and draws level with her hand. (00:20:05)
Continuity mistake: While Clark and Zod/Lex are fighting in the field, the blood on Clark's right cheek changes throughout the scene, from disappearing, to smudging, to thick blots. (00:30:00)
Answer: The kryptonite only creates mutations when it reacts with the environment and an individual. Remember, the FDA and several environmental groups did check out Smallville after the meteor shower, but found no danger in the rocks. Obviously they're not always emitting mutating rays, or all the flora and fauna of the surrounding area would be horribly mutated.
Phoenix