After Abby kisses Lana you can clearly see her necklace. Earlier it was shown that Clark found that necklace in the shower drain. Either Abby has multiple necklaces or this scene was shot before the shower one.
In the opening scene as the camera pans down to Lana's necklace while she's in the Dunk Tank, Kristin Kreuk glances directly into the camera, unaware that she's still in the shot. This is at least the second time she's made this mistake. The first time was in Season 3's episode, "Slumber."
Lex and Jason introduce themselves to each other, despite the fact that they used to play together as children, revealed in the episode "Veritas".