Audio problem: When Clark talks with Lex in the house where he found the two dead bodies,just after Lex finished his phone call, Clark says to Lex, "You looked at me straight in the eyes and told me she was dead," and then the shot changes to Lex. Clark's lips are still moving even though his line is over.
Audio problem: At the hospital Clark and Jason talk about Clark possibly being on steroids. Watching Jason's mouth reveals he actually says "and a college career." before he says it out loud a few seconds later.
Audio problem: In the scene where Jason and Isabelle/Lana are talking and he is about to burn the book with the lighter, she quickly grabs the book with her magic. Isabelle then says some sort of spell flinging Jason against the wall to the right. In the shot where you see him thrown against the wall, Isabelle has already said the spell, but you can see her mouth moving saying the same thing (though nothing is heard).
Audio problem: Clark is upset over the death of Alicia and is in the barn loft bouncing a ball off the wall. His parents come up and Jonathan sits next to him and tells him that what happened was not Clark's fault. The very next shot shows Martha sitting down and you hear Clark throw the ball again but you can see that his hands never make and motion to correspond with that action.
Audio problem: As the band is playing right before Clark and Lois enter the prom, the drums are not synced with the music.
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