Continuity mistake: When Clark's truck stops rolling over the hood is stuck open. As Clark pops up from the front seat the hood is down for the camera to get a shot of Clark.
Continuity mistake: Clark is poisoned by a silver kryptonite splinter. When Clark is stabbed, the splinter is in his left index finger. Then, in the scene with Fine after class, (less than twenty minutes later,) Clark is seen rubbing his right index finger.
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