While talking to Nate in the park Chuck offers to give him some of his mother's medicine. It is revealed later in the series that Chuck's mother died shortly after Chuck was born.
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.
When Dan, Serena and Jenny is leaving the Kiss on the Lips party Serena jumps into the taxi before Dan does. However, when in the taxi Dan is seen sitting between the two girls.
When Nate and Chuck are talking about why Nate hasn't had sex with Blair yet, Chuck jokingly says he'll swipe some of his father's Viagra or his mother's Paxil. His mother is not alive.
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).
In the Pilot, Nate and Chuck are seen to be riding the public bus with Dan. The bus travels from the other side of town, and yet they are known to live hear the school. In the next few episodes Chuck is seen to be driven by his chauffeur in a limo from and to school.