(At around 22 minutes) When Specs and Tucker enter Elise's house, there is an African Tribal painting hanging on the wall. It is the same painting seen hanging in Daniel's study in Paranormal Activity 2 (2010) and in the Grandmother's house in Paranormal Activity 3 (2011).
There is an Australian flag in the corner of Dalton's room in the Lambert house. Leigh Whannell (Specks, writer of Insidious (2010)), Rose Byrne and James Wan (writer and director of Insidious 2 (2013)) all come from Australia (however, Wan was born in Malaysia, but later moved to Australia).
(At around 34 minutes) When Renai is walking down the hallway, the camera follows her. As the shot moves, archways on the right reveal parts of the dining room and living room. In one of the archways, the ghost woman who attacks Renai can be seen sitting in a chair in the corner surrounded by a window. When she passes the next archway, the woman is gone.
This is the third movie and second project (counting the two movies as a single franchise) that involved horror and Barbara Hershey, after The Entity (1982). In it, Carla Moran (Hershey) is pursued by an evil being. Curiously, in this movie, her son, Josh, played by Patrick Wilson, is pursued by an evil being.
In this movie, as well as its predecessor, Insidious (2010), Josh Lambert (Patrick Wilson) is pursued by ghosts who want to invade his physical body. This is the opposite way in A Gifted Man (2011), where Michael Holt (Patrick Wilson) is capable to see Anna Paul's ghost (Jennifer Ehle), in order to better him as a doctor and a person.
The trivia items below may give away important plot points.
During the last scene of the film, as Specs and Tucker arrive at the door of a family in seeming distress, a little girl sees Elise now in her ghost form. As Specs and Tucker explain to the parents who they are, Elise goes inside the house and approaches another girl who is in an apparent coma, and begins to speak to her. She then hears a sort of rusty crackling noise and when looking behind the girl, sees something horrible and gasps. The noise she hears is that of the Lipstick Face Demon from the first movie, moving his claws. For a long time before the third chapter was announced to be a prequel, fans who understood this were excited to see the return of said entity of the further, making a return for the third film.
As the first film was inspired by Poltergeist (1982), this film also shares some similarities with Poltergeist II: The Other Side (1986). In both movies, the father of the family becomes possessed by the evil spirit of an elderly madman.
Josh (Patrick Wilson) and Tucker (Angus Sampson) also star together in the second series of Fargo (2014). However, unlike Fargo (2014), they share just one scene together in this film, and they never actually see each other face-to-face or exchange any dialogue.