Was supposed to be the series finale. Eric Kripke initially planned for Supernatural to be five seasons long. That is exactly why the episode is named "Swan Song" (goodbye) with a voice-over and flashbacks. But Sera Gamble, the new show-runner, took over penning it from season six and on.
When Dean says to Sam "you're a grown - overgrown - man", Jensen, messing around on set as the Supernatural actors tend to do, ad libbed the word "overgrown" in an attempt to make Jared (Sam) laugh. The ad lib made it into the final cut.
When Lucifer is showing Sam all of the people in his life who were really agents of Azazel watching over him, he refers to Sam's prom date Rachel. Dean is said to have slept with Sam's prom date Rachel Nave on prom night, while in his Djinn-induced dream.
Stull Cemetary, where Dean found Lucifer and Michael, is an actual place about five miles west of Lawrence, KS. It has been rumored that in the past black masses were performed in the chapel ruins and that on Halloween night, a demon or devil sat in a tree inside the cemetery and answer questions about your future.
Sam, talking to Dean in the car, refers to jumping into the hole with Lucifer possessing him as a "Triple Lindy." This is a reference to the '80s hit film Back to School (1986) starring Rodney Dangerfield. The Triple Lindy is an "impossible" dive performed by the main character at a dive meet, which helps the team to win the meet, saving them from what would have otherwise been a crushing defeat.