The reason Detective Barry Frost died suddenly was due to actor Lee Thompson Young's suicide prior to the filming of the 5th season.
Several articles have stated that the tears at the end were real. The actors used Frost's death to say goodbye to Lee Thompson Young. So very sad that he died at such a young age.
When Frankie and Korsak start discussing a car, Jane asks, "Click and Clack, is there a point?" Click and Clack, the Tappet Brothers, are the nicknames for Tom and Ray Magliozzi, the hosts of the famous Boston based radio program "Car Talk."
The pajamas and robe Jane is wearing in her dream are the exact pajamas and robe that Angela is wearing when she comes in to get coffee while Maura and Jane are talking at Maura's house.
Many of the episode titles are also the names of songs; "A New Day" is by Kenny Eldridge. ("New Day" is a song by Alicia Keys and "A New Day Has Come" is a song by Celine Dion.)