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.
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.