Continuity mistake: Season 7, episode "Persona". When Lana walks in from the balcony her hair swaps from being in front of her shoulder to behind it, then back again. Then when she sees Bizzaro's face in the reflection of the glass, her hair is styled differently again.
Continuity mistake: During the scene where Clark and Knox fight, Knox smashes his blue container with his weapon. Shortly after this, there is a shot where he is looking in horror at what he'd done and behind him, Chloe is struggling with her ankle cuffs. She appears to be wearing shoes in this frame, whereas before and after, she is barefoot.
Continuity mistake: When Lana comes in with her brown bag of groceries, she puts the vegetables and wine bottle on the counter. During the rest of the scene these items move their positions a few times.
Continuity mistake: When Kara and Clark get off the elevator, there's no-one around them as they're walking away. In the next shot of Kara and the elevator, 3 people have suddenly gathered to look at the broken elevator. The elevator was off camera for less than 2 seconds, not enough time for 3 different people to get into the position they're in when they weren't close by to begin with.
Continuity mistake: When Phantom Clark finds Lana on the balcony with her phone pressed against her ear, she lowers the phone down to her waist twice between shots.
Continuity mistake: When Lex starts learning about Veritas he sees the symbol on his office window, but that was never there in all the 6 previous seasons.
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