Continuity mistake: Lois hands two boxes of decorations to Clark for Chloe's party but in the next shot he is holding just one box. The other box reappears when they turn to go into the barn.
Continuity mistake: When Lex comes into Lana's place to confess that he was the reason Jason got fired, we firstly see Lana putting some things into a cardboard box. Then during the following shots, the items visible at the top of the box keep on changing between shots.
Continuity mistake: While Clark is chained up, Chloe uses her fingernails to dig into Clark's chest. You can see that since Clark has had his powers taken away her fingernails draw blood. In the shot of Clark screaming in pain you should see where the blood came from the scratches but there is no blood visible.
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