Deliberate mistake: Lindsay is not at Caleb's funeral in the last episode of season 2, "The Dearly Beloved". It's unlikely that she would have missed it - she is still on good terms with him and he is her father, the only reason she left was to be with her mother, not because she didn't love Caleb.Obviously they didn't want to create another drama with having Lindsay there while they had to focus on the important story-lines - Caleb's death, Kirsten's drinking and Marissa shooting Trey.
Factual error: In the first episode of season two you see Teresa with Ryan, yet she has no bump. Season one ends at the end of the school term and season two starts off just before the school term. Therefore there's been about two and a half months of school holiday plus the time between her sleeping with Ryan and discovering she's pregnant and Ryan leaving for Chino with her - therefore she should be showing some signs of pregnancy in early season two.
Factual error: How exactly does Seth unlock the door? The entire purpose of locking the storeroom is to keep mall customers out so they won't steal things, not to keep people in there, so there is no way it was only lockable from the outside. If it was a key operated lock, why would someone just leave their key in the keyhole for someone to take? There is no explanation for Seth being able to unlock the door.
Character mistake: Marissa said to Ryan (in episode "The Brothers Grim") that the last time she was in Chino, she "saved his ass". She was referring to Thanksgiving, but that's not the last time she was in Chino. The last time she was in Chino was in "The Nana" when she ran there after finding out about Luke and her mother's affair.
Continuity mistake: At the beginning of the episode, Kirsten and Ryan are sitting at the table. From the front you see Kirsten pick up her coffee cup. In the behind shot you see her put it down and in the next shot, which is from the front, the cup is still in her hand and she's putting it down again.