Continuity mistake: When Zod/Lex pierces Lana's hand, he pierces it at the same level as Lana's head. But in the camera angle immediately following, Lana's head slide downs the wall and draws level with her hand. (00:20:05)
Visible crew/equipment: Martha and Lionel are in the barn when everything starts to fall apart. The camera shot changes from them to a boat falling from the rafters. As it falls there is a visible production crewmember hiding next to the window, holding a hay bale. (00:28:15)
Continuity mistake: While Clark and Zod/Lex are fighting in the field, the blood on Clark's right cheek changes throughout the scene, from disappearing, to smudging, to thick blots. (00:30:00)
Continuity mistake: In the scene between Martha and Clark near the end of the episode, the cut on Martha's forehead switches from above her right eyebrow to her left, and then back again. (00:37:40)
Factual error: The last scene is said to be set in "Iguazú Falls - Patagonia, Argentina." Iguazú Falls are nowhere near Patagonia. (00:40:00)
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