Continuity mistake: When Lucas is in the Luthor mansion talking to Lionel and Lex, he picks up and inspects the samurai sword, then returns it to the holder, hanging it on one peg by the strap. In the next immediate shot, it is resting between the pegs, as it was before he picked it up.
Continuity mistake: The motorcyclist has his visor down when he appears at the end of the alley. He raises the gun, then reaches up and lowers his visor again. (00:27:00)
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