Continuity mistake: After Greg hits Clark with the pipe in the foundry, Clark's hands are very dirty. In the closeup on Clark's hand immediately after he lands, the hand is clean to show the effects of the Kryptonite. (00:39:45)
Continuity mistake: Despite the fact that Clark and Jonathan are looking in the rafters above them for Greg, their flashlights stay relatively level. (00:26:35)
Continuity mistake: While researching Greg with Chloe, Clark changes from one hand on the computer mouse to arms crossed and back. (00:34:00)
Continuity mistake: Clark's hair goes from long to short and back to long as Clark, Pete and Chloe talk about Greg's tree fort.
Factual error: When the bug kid crashes his car, he's driving a VW Beetle, which has the motor in the rear, therefore would not have shut off in a head on collision.
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