Continuity mistake: As Chloe hurries to put Justin's art book back before Justin comes in there is a piece of white paper sticking out of the top and bottom, based on the way she put it back, of the folder. When Justin sees the folder has been moved there are no papers sticking out.
Continuity mistake: When Clark goes to Kwan's house the set of pathway lights on the left, the side where Justin pulled only one out, is gone. Earlier the lights ran down both sides of the driveway.
Continuity mistake: As Justin turns the spike end of the pathway light towards Principal Kwan, in the close up, the light is at a height that is just lined up with the light in front of it. After the shot change, the shot simply zooms out to include Justin on the right side of the screen, the light is now several feet higher than it was before.
Continuity mistake: When Chloe is kissing Justin the shot is behind him and her hands are not on his neck. The shot switches to behind Chloe and she does have her hands on his neck.
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