The Desperate Kingdom of Love - S2-E1
Audio problem: In the flashback, when Hayley and Nathan are in bed together and it's raining outside, she says, "Well, maybe so," but her mouth is not saying those words. She smiles and closes her mouth.
The Truth Doesn't Make A Noise - S2-E2
Continuity mistake: Early in the episode, Lucas is listening to his Ipod on the river court. Later, Keith comes to his house and tells Karen that Lucas left it in his truck.
Revealing mistake: Brooke takes the towel off Nathan, but you can tell that he is not actually naked and is wearing boxers.
You Can't Always Get What You Want - S2-E4
Revealing mistake: When Dan and Lucas are racing Felix, from one scene to the next where the actors are replaced by stunt doubles. Close up, they are Paul Johansson and Chad Michael Murray, but from far away when the actual driving is happening, they are not the same people.
The Hero Dies in This One - S2-E13
Plot hole: When Dan asks Lucas how he managed to lie to Karen about his heart test results then successfully move out, Lucas tells Dan that he showed Karen Keith's test results letter instead. In a prior scene, Lucas hands Karen the results letter and she is elated to read no Hypertrophy issues. Being a very savvy business woman, Karen would have noticed the addressee on the letterhead was Keith, NOT Lucas.