Continuity mistake: When Christina Hendricks is about to leave the office, while talking to Don, she is wearing a scarf on her hair. The scarf corner is alternatively tucked in her coat collar and then outside, and inside again etc. in alternate angle shots of the scene.
Continuity mistake: Don is called to Bert Cooper's office to discuss Hilton coming into the office earlier that day and his (Don's) lack of a contract. Right after Don enters Bert's office, Bert asks, 'when were you going to tell us about this (Hilton)? ' and you can see the contract (with a blue cover) sitting on the table in front of Lane. There are several more shots of Lane and Roger as they discuss the contract, and it is sitting in the same spot on the table. But after Lane explains that Don must sign the contract, he pulls it from his jacket (not the table) and hands it to Don.