Continuity mistake: As Clark and Geoff are driving, the first shot of the SUV as it comes around the corner shows the ground is wet and the wipers are going. During their conversation the windows are constantly being rained on. Geoff points to the stadium and they arrive a few minutes later and as they drive into the garage you can see the street is dry.
Continuity mistake: After Lois and Clark find out that Cooper dies, and after Lois leaves, while Clark and Geoff talk about how Lois is trouble Geoff hair keeps going from neat to slightly messed up. When the camera is behind Clark, Geoff's hair is all shaggy on the left side of his face. When the camera is behind Geoff, his hair is fixed so that the loose strands are tucked behind his left ear. These changes repeat throughout the scene.
Factual error: While Clark is standing on the field at Metropolis University at the end of the episode the shot pulls back to show the entire field. As it does there are two 50 yard lines visible and the goalposts in the wrong position because it's a Canadian football field, but Metropolis University is in Kansas.
Factual error: Clark finds Lois in the sewer and decides to use his heat vision to destroy the lock of the gate that is separating them. Clark uses his heat vision on the lock and the lock explodes, this is a very minor explosion. It also causes the door to fly off the hinges. Now the explosion from the lock is odd since heat vision causes things to melt and not explode, but Clark wasn't even using the heat vision on the hinges and they are destroyed. This is inconsistent with how his heat vision has been shown to work before.
Other mistake: When Clark is in the girl's room and a noise comes from the closet, Clark uses his X-Ray vision to see there's a person in there because he can see the skeleton and is surprised to see it's Lois when he opens the door. However, it's been shown before that Clark can use his X-Ray vision to see objects and people through solid objects. He should have been able to see that it was Lois rather than just a skeleton of an unknown person.
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