Continuity: While talking to Nate in the park Chuck offers to take some of his mother's medicine. It is revealed later in the series that Chuck's mother died shortly after Chuck was born.
Continuity: While Nate and Chuck are talking in the park, Nate's collar changes between under his bag strap to out from under his bag strap, in what should be 2 seconds, this happens several times.
Plot holes: When Serena and Blair meet at the Palace bar on their first day back to school, Serena asks Blair how her mom is doing "with the divorce and everything". At this point in the story, Blair's parents are not divorced yet. Blair's dad asks for a divorce much later in the series (Thanksgiving of the 2nd season).