Continuity mistake: The posters on Seth's walls are different to the ones he had up when he left Newport. Since Seth has been in Portland all summer and not returned home, it's impossible for him to have changed them and extremely unlikely for Sandy or Kirsten to have either.
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.