Continuity mistake: When Lionel and Lex are talking, after they sit down, near the end of the conversation, Lionel asks who Lex's source is. Lex is looking straight ahead. In the next shot, we see Lex turn his head to face his father. In the next shot, Lex is looking straight ahead again.
Continuity mistake: When Ryan gets hit by Martha's car, his comic book goes flying out of his hand. When he falls to the ground, he's holding his comic book again.
Continuity mistake: When Lana goes to see Lance, who's packing up the serum, you see 5 syringes strapped in, 1 with serum in it. When Lance hits Lex and his case falls open, you can see the 5 syringes still strapped in, but now none have any serum in them. When Lance goes to pick up the case, only one syringe is strapped in, and it's in the wrong spot, next to some blood, and one syringe out of the strap lying in the open case.
Continuity mistake: In the shot of Lana being stepped on by the horse, her right arm is out by her side and her hair is covered with the wood shavings lining the stall. In the next shot of her, her right arm is up so that her right hand is by her head and her hair isn't covered with the shavings. Her arm is back out by her side in the next shot when Clark arrives.
Continuity mistake: When Lex and Clark go to where Lex saw Morgan and it's a bunch of people sewing, Lex grabs a man's hand and puts it on the table. On the table is a piece of fabric that the hands are on, but in the close up shot, there is no fabric. It's back in the next shot.
Continuity mistake: When Perry arrives at the Kent's, and they start talking, Clark has his hands in his pockets. After being asked it he's a report, Perry starts to walk around and the shot changes and Clark no longer has his hands in his pockets and is then seen putting them in them.
Continuity mistake: The "Go Crows" sign Lana is working on changes depending on the camera angle. From the front you can see the! at the end. In the close up shot of the sign, the! is now gone. When Lana underlines "crows" it's back but the writing is slightly different.
Continuity mistake: When Lex has the envelope and pulls it away from Lana as she grabs for it, the flap is opened, so the wax seal is on top. In the next shot the flap is folded down.
Tempest (1) - S1-E21
Continuity mistake: The way the pillar is on top of Lionel changes in the scene. In the first shot, he's on his side and it lands on his hip. In the next shot he's now on his back and the pillar is going somewhat across his body. In the wide shot of him as the roof falls, the pillar is below his belt, almost perpendicular to his body. Then back to being on his stomach and part of his chest.
Continuity mistake: When we first see the meteorite tattoos on Wade Mahaney, they are all green. When Clark sees Whitney's tattoo, it is plain black. It was also faded, which means it's not a fresh tattoo. Whenever Clark runs into the other guys, he feels the effects of the meteor rocks, but never when he is alone with Whitney.
Continuity mistake: When Phantom Clark finds Lana on the balcony with her phone pressed against her ear, she lowers the phone down to her waist twice between shots.
Continuity mistake: In the previous episode, "Slumber," Clark wrecks the farm truck by falling asleep while driving. The car has massive damage that, at a minimum, would be extremely expensive and time-consuming to repair. Realistically, though, it is completely totaled. In the episode "Perry," the truck is again in brand new condition, like it was untouched.
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