Continuity mistake: In season five, when Blair is in the therapist's office with Chuck, she states that her dad left when she was fifteen. However in Blair Waldorf Must Pie in season 1 it shows that Harold was still with them last thanksgiving, when Blair must have been 16, because her birthday is before thanksgiving.
Continuity mistake: When Dan and Jenny are discussing what Rufus and Lily are doing in Boston, Jenny picks up her coat and starts straightening it out before putting it on. The shot changes and she has instantly moved 90 degrees clockwise and is now seen removing her coat. When shot changes again she is suddenly back to her original position and has resumed putting on her coat like she was doing in the first place. (00:01:52)
Continuity mistake: Before Chuck arrives at Sotheby's, Nate is walking down the street. There's no continuity with the passers-by behind. Check the woman in black with a pearl necklace. She quickens her pace and overtakes him, but in following shots she is still walking behind him.