At the beginning of this episode Lana is supposedly naked. However, when she dives off of the pier, she is wearing a pair of red shorts.
Just moments earlier in the episode, Clark and Lana are skinny dipping and both have their hair visibly wet when they hear a woman scream. When Clark gets to her, his hair is perfectly dry.
While there was a close up on Clark while he was asleep, you could see a small cut right above his eye, which is impossible if he can't be hurt.
Lana and Clark decide to go skinny-dipping. As Kristin Kreuk is taking off her clothes behind Tom Welling, the camera pans from left to right over Tom. At the last moment, Kristin looks at the camera, obviously unaware that she's still in the shot.
When Lana is supposed to be naked, she and Clark are just about to kiss, you hear the scream and you can see a blue strap of a swimming costume.