Revealing mistake: There are times when it is quite evident that Michael Rosenbaum is voluntarily bald.
Continuity mistake: When Clark is at the table, when Lois is serving him dinner, she pours his beer into a glass and there's about 2 inches of foam on top, then it switches to a shot behind Clark and there's almost no foam on the beer. (00:09:10)
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