Continuity mistake: When Clark first finds the necklace in the shower drain it is all in a single line with no loops or overlaps except at the very end. As Clark picks the necklace up it now has several loops and overlaps all along the chain and not just at the end where the clasp is.
Continuity mistake: When Chloe is first asking Pete and Clark the questions for her poll you can see that the top page on her clipboard is yellow. Then she shows it to Lana and the camera gives you a nice big shot of the front piece of paper and it is white. When Chloe says "its unleashing the inner Lana." the front of the clipboard is shown again and now the paper is yellow.
Continuity mistake: As Morgan Edge hands Lionel the bag with the mason jar of blood the camera is on Lionel's side and bag has a very neatly folded flap to keep the top closed. As Lionel takes the bag, while the camera is on Edge's side, the opening is wrinkled and not folded shut as it was before.
Revealing mistake: Lana and Clark are supposed to be skinny dipping at Crater Lake. When Clark pops up out of the water and goes "boo" to Lana she spins around and splashes Clark for making her worry. As she starts to splash Clark her body comes out of the water far enough to see she is wearing a tan tube top instead of actually skinny dipping.
Plot hole: Clark is crippled throughout much of the episode due to there being kryptonite in the tunnels. At one point he gets a rock stuck in his arm and he bleeds from this wound. For much of the time after this he has blood running down his arm from this wound. After he gets out of the tunnel he is healed but the problem is the blood from the wound also disappears. The actual wound should have healed up but the blood from that wound should still be shown staining his arm.
Continuity mistake: When Lana was making a video to send to Whitney, we see her interviewing Clark. We first see Clark through the camera's view finder. Lana then drops the camera (pointing it down) such that we see Clark's feet. Cut to a different angle but now Lana's is pointing the camera at Clark's chest level.
Continuity mistake: Jonathan reveals the kryptonite to Clark/Dawn and Clark starts to react to it. Because of the kryptonite Clark drops the tiara he took from Chloe and it hits the floor near Chloe's hand. Then, just before Dawn is forced out of Clark's body, the tiara has slid several feet away and is now close to the furnace. Clark turns his attention to Chloe, she is faking an injury at this point, but now the tiara has moved back to be near her hand.
Continuity mistake: In the barn loft evil Lex has a kryptonite ring in a lead box. When the camera is in front of Lex he is holding the box so that the lid is pointed towards his thumb. When the camera is behind Lex the box is turned so that the lid is pointed towards the end of his fingers.