Factual error: When Clark is on the roof, and Lionel calls him over to walk to his car with him, in the background as they walk, you can see a building which has the logo of a Canadian bank. Although set in Smallville and Metropolis, Smallville is filmed in Vancouver.
Other mistake: When Clark looks at Oliver's newspaper clipping, there's a story about an AIDS hospice being saved. But the 2nd paragraph on talks about the 2nd meteor shower to hit Smallville that has nothing to do with the hospice being saved.
Continuity mistake: After Lois pins the sketch of the Green Arrow, the pushpin changes position. In some shots, it's across the top of the paper and in others it's in between the top of the sketch and top of the paper.
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