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 Clark looks over to Lana, while in the coach's office, she starts to pull up her right sleeve that had slipped down. As Clark says "Lana.what are you doing here?" the shot is behind Lana and her sleeve is down and is showing a lot of her upper arm and shoulder but when the shot switches back to the front of Lana, she has her sleeve pulled up covering most of her upper arm.
Continuity mistake: When the mirror falls on Lana and shatters on her, you can see that there are bleeding cuts on the left side of her face. However, for the rest of the episode, there are no cuts on the left side of the face (where they should be) but there are cuts on the right side of her face.