Thanks! My first post on AV Club and I get the nod from the author! Ha. I wasn't sure if this had been brought up in the comments so far but it was one of the first things I picked up, probably because I've been doing a lot of re-watching of early seasons and it was fresh in my mind. 

Another callback that I am not sure was that obvious was Peggy saying to Don to "move forward". Weren't these the same words that Don said to Peggy years back when he visited her in the hospital after she'd had her post-partum breakdown? "Get out of here and move forward. It will shock you how much this never happened"