Continuity mistake: We discover that because time is non-linear in the Further, it was Josh's spirit who was the one knocking on the door in the original film. However, the knocks we see Josh make in this film don't quite match up with the original film. In this film, he knocks many times frantically, whereas in the first film, we only hear a few faint knocks in rounds of three. (The film tries to mask this by inter-cutting footage from the first film to illustrate the point, but it still doesn't quite match up).
Trivia: During the opening of the film, Lindsay Seim portrays the younger Elise in the flashback sequence. However, Lin Shaye, who plays the older Elise, dubbed over Seim's dialog in post production. Director James Wan stated that this was done because Shaye has such a unique voice, and thus, it allowed her presence to be felt early in the film.