Factual error: When Palmer is looking at the phone log, the wrong area codes are shown. Most of the list consist of the city of Topeka, with Wichita and Kansas City also seen, with a "206" area code. "206" serves the Seattle Washington area. The other area code seen is "620", which is a Kansas area code, but Topeka is in the "785" area code.
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.
Other mistake: When Ryan gets hit by Martha's car, you can see he has both shoes on as he's going to the ground, and we never see a shoe flying through the air during the hit. Then James and Debra find the shoe in the middle of the street as they watch Martha drive off.
Visible crew/equipment: When Jeremiah has just stabbed Clarke in the barn, after he goes down the steps and the camera follows him walking away you see the back of one crew member's head and then a second crew member's, starting in the bottom right of the screen. Just before scene cuts to black you see the first crew member turn his head to the left. (00:26:46)
Revealing mistake: When Pete is on his motorbike they pan to his face and we can see the stunt double. (04:36:00)
Visible crew/equipment: When Martha and Jonathan are talking about Lionel's job offer in the kitchen, a boom mic can be seen in upper left corner of the screen.
Revealing mistake: When Clark gets hits by lightning whilst trying to save Eric, you can see that Eric is a stunt double.
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.
Other mistake: After Clark enters the phone booth, he begins to change out of his red shirt. The phone booth is mostly frosted glass, but it is possible to make out that Clark only becomes bare-chested before the next shot when he comes out of the phone booth with the blue shirt on. (00:02:14)
Continuity mistake: When Lex starts learning about Veritas he sees the symbol on his office window, but that was never there in all the 6 previous seasons.
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