Continuity mistake: When Jodie hits the deer her vehicle spins out. As it is spinning the headlights are not projecting any kind of visible beam. After the car stops, and she is facing the deer she hit, the headlight is now projecting a very bright beam to illuminate the deer on the road.
Continuity mistake: When Sean emerges from the lake and goes to the campfire, his clothes are completely dry despite the fact that he was totally underwater for hours. (00:04:50)
Continuity mistake: At the end of the episode, the doctor takes the mouthpiece out of Lex's mouth, then the scene cuts to Clark. When it cuts back to Lex, the mouthpiece is back in his mouth.
Continuity mistake: When "Lex" threatens the bank manager, he points the gun at his face, but the next shot on the manager shows the gun at his throat. (00:01:35)
Continuity mistake: As Victoria is being drowned in the bathtub, she appears very deep in underwater shots and only a few inches down in overhead shots. (00:26:00)
Continuity mistake: As Harry goes into the pond the second time the pylons are connected now with yellow crime scene tape when the camera is out over the water looking at Harry on the walkway. As the camera is in the water, watching Harry fall in, the pylons are gone, even though they should be in the shot, and are connected by a wire instead of crime scene tape.
Continuity mistake: When Sean is in the nurses office, having his temperature taken, the light on his left side is on. After the nurse wants to retake the temperature the light is now off.
Revealing mistake: In "Rogue," just after Jonathan has been arrested, Clark walks into the house and punches a wooden beam. If you look closely, you can see the crack in it where it has already been broken. (00:26:35)
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