When Jack's condition takes a turn for the worse, Sam, Dean and Castiel bring him to the Emergency Room. When speaking with the nurse she asks for his birth date. Dean looks confused, but Sam quickly answers "May 18th". May 18, 2017 is the date the season 12 finale episode "All Along The Watchtower" aired, where Kelly Klein, gives birth to Jack.
Frank Kellogg's house is the same house used in episode 4x11 Family Remains.
Frank Kellogg's house, where Nick goes to question him, is the same house where Rev. Roy La Grange lived in Faith (Season 1, Episode 12). The same house was used many times as a location in Supernatural and other shows.
Nick mentions an ex-neighbour called Artie Nielsen. This is the name of Saul Rubinek's character on Warehouse 13.