Continuity mistake: After Clark saves Lex from the ring of fire we see the area that was burned by the fire. The candle that Shannon used to light the alcohol is no longer on the floor. Since the candle was in the fire it should be partially melted and there would be some evidence of it.
Audio problem: Clark says "What if it's some sort of high tech crop duster or something?". Pete replies with "She'll never let us live it down." After the shot changes you can see Pete's mouth say "live it down" again with no sound coming out.
Continuity mistake: As Chloe and Clark talk about their feelings the shoulder protector/pad of Clark's backpack switches position with no help from Clark. At first it is more in front and showing more than a few shots later, again with no help from Clark by shifting the backpack, it is more on top of the shoulder and not as visible.
Deliberate mistake: When Clark is chasing Emily at light speed, Clark decides to use heat vision to knock the water tower down. The water tower shouldn't have had enough time to fall down and intercept Emily, seeing as she's travelling too fast.
Continuity mistake: When Lana hands the gun to Jimmy, she hands it to him aimed away from her. When Jimmy takes it, it is aimed towards her. (00:59:00)
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.
Transference - S4-E6
Continuity mistake: As Lionel's cellmate tells Lionel "I've always kind of had a thing for pattern recognition." the shot changes mid-sentence. Between changes Lionel's cellmate goes from having his arms crossed, hands holding his elbows, to having his hands almost clasped together.
Continuity mistake: Lex and Clark are playing pool. Lex misses his shot on the 12 ball and scratches which leaves the 12 ball near the pocket. After Clark gets the cue ball, he makes his shot on a different ball and the 12 ball that was next to the pocket is gone without it being knocked into the pocket.
Continuity mistake: Clark tells his parents that he went to see his grandfather and that he asks to give the cashiers check to his mom. He takes it out of his pocket with his right hand and Martha Kent takes it with her left hand. Cut to a different angle but now it is Martha's left arm which is see being pulled back to her body as if she took the check using her right hand instead.
Continuity mistake: In the flashback when the Kent's meet Clark, we see Jonathan loading the space ship onto the back of the pick up truck. He then see Martha carrying Clark who was wrapped up in a blanket. At first we see that Martha's left arm is wrapped around Clark's back onto his shoulder. Cut to Jonathan's viewpoint and now Martha is just supporting Clark and her left arm is no longer around Clark.
Continuity mistake: At the Talon, Clark tries to write his essay when Crissy shows up asking Clark to sign up for the decorations committee. Crissy holds her clipboard with her left hand and places her right hand on her hip. But in the next shot Crissy's right hand on the clipboard as well. Cut back to the previous angle and her right hand is back on her hip.
Continuity mistake: When the weather vane falls to the ground the rooster portion of the vane gets bent. When Jonathan picks it up and starts to angrily walk away from Martha and Clark it is straightened.
Visible crew/equipment: When Prof. Joseph Willowbrook and Lionel Luther walk down the steps of a building, talking about the possible whereabouts of Jeremiah Holdsclaw, they begin to cross the courtyard, which is bathed in sunlight. As they walk across, and eventually walk under another building, as they cross the courtyard, you can see the shadow of a large reflector board moving along with them as they walk.
Audio problem: When Sean flags down Whitney's truck, we hear his brakes squeal as he slams on the pedal. A few quick cuts later we see him do the same thing, apparently for the first time. (00:33:25)
Continuity mistake: When Sean emerges from the lake and goes to the campfire, his clothes are completely dry despite the fact that he was totally underwater for hours. (00:04:50)
Continuity mistake: At the end of the episode, the doctor takes the mouthpiece out of Lex's mouth, then the scene cuts to Clark. When it cuts back to Lex, the mouthpiece is back in his mouth.
Continuity mistake: When "Lex" threatens the bank manager, he points the gun at his face, but the next shot on the manager shows the gun at his throat. (00:01:35)
Continuity mistake: When Clark's parent show up at the house Clark hurries up and acts busy. He grabs 4 boards and moves them from one part of the kitchen to the other and leans them up against the wall. When Clark turns around to face his parents there are only 2 boards leaning on the wall.
Revealing mistake: After Lana is warned by Chloe that Clark has gone all 'Captain Paranoid', she examines the silver 'meteor rock' more closely. We see a shot of her hand holding the 'meteor rock' but the hand that is shown is obviously not Kristin Kreuk's hand.
Plot hole: In the episode Onyx, Lex's alter ego holds him hostage in the wine cellar claiming that when they were kids, Lex accidentally got locked in down there. In the pilot episode, it is established that although Lionel had the castle shipped over from Scotland brick by brick and built, they never moved into it.
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