When Castiel and Metatron are in the car, the voice on the radio is Robert Singer, executive producer and oftentimes director of Supernatural (2005).
Charlie tells Sam, "He's right. Let's get our Alan Turing on. Decrypt this bitch." Alan Turing was a British computer scientist, mathematician, logician, and crytanalyst who played a key role in deciphering intercepted encrypted messages that helped the Allies defeat the Nazis in key battles and shortened World War II by several years. Benedict Cumberbatch played Turing in the 2014 film "The Imitation Game."
In the end credits the following appears at the beginning of the credits,"In memory of Jaap Broeker 1950 - 2015" Jaap Broeker was a stand-in in Season 2 Episode 14 - "Born Under A Bad Sign"
Charlie calls Jacob Styne "Gambit." Gambit is a mutant superhero from the Marvel Comics X-Men series who hails from Louisiana and speaks with a thick Cajun accent.
Metatron references Alanis Morrissette's song, "Ironic," twice. Alanis Morrisette played God in the Kevin Smith film, Dogma (1999), which was about two rogue Angels trying to get back into heaven. The angel Metatron was also featured in this film, played by Alan Rickman.