The bumper sticker on Ajay's taxi reads "HOW'S MY DRIVING ? CALL 1-555-GO2-HELL"
As of this episode, both Winchesters have been to Hell, Heaven and Purgatory.
Garth's boat is named "Fizzles' Folly". Mr. Fizzles is Garth's sock puppet from the episode Party On, Garth (2012).
When Sam was trying to convince Bobby that he wasn't a demon, he was referencing the episode You Can't Handle the Truth (2010) when Bobby told Dean about being a fan of Tori Spelling and getting a pedicure.
Ajay is the first known reaper to go rogue. It is unknown why he turned against Death, or if Death even knew about his disobedience.