Continuity mistake: When Blair is walking between her two minions, she is holding green files and Zoe, the girl to the left (her right), is holding red files. After Serena enters the scene, the shot changes and Blair is now holding red files, while Zoe is holding the green files. (00:01:22 - 00:01:48)
Character mistake: Rufus says "That was before I found out you raped my 14 year old daughter." referring to Jenny. Jenny, in previous seasons is 15. Jenny was born in 1993, and season 4 was filmed in 2010, making Jenny 16 at least. (00:25:35)
Suggested correction: In this scene, Rufus saying, "my 14 year old daughter..." was referring to Jenny's age at the time Chuck tired to force himself onto her (beginning of the first season), not her current age in season 4. Jenny was 14 in the beginning of season 1.