Continuity mistake: As Lana says "I was thinking the same thing but we don't seem to be very good at it." there is a camera change as she says "but". Before the change she only has the red cup on the tray but after the change, even though she didn't pick it up, she has a red and a yellow cup on the tray. The other cup was on the table and jumps to the tray between shots.
Continuity mistake: When Chloe crashes through the bridge rails her windshield is dotted with raindrops. Just a few seconds later the blue truck hits her car from behind and now her windshield is dry.
Continuity mistake: Lex whispers into his father's ear something about keeping secrets. At first the view is of him less than an inch away from his ear. In the next shot he's at least six inches away.
Factual error: Clark lifts Chloe's car by the back. He is only holding on to the very back left portion of the car, yet while he's turning it, the car is completely horizontal. The front of the car would buckle since it weighs so much and isn't being handled by its center of gravity.
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