(at around 1h 10 mins) During the climax, in the storage area with all 101 dolls, you can see Jigsaw's doll from the "Saw" films sitting on the floor, and Edgar Bergen's doll Charlie McCarthy on one of the shelves. The doll that Detective Lipton throws over his shoulder in this scene is a replica of ventriloquist Jimmy Nelson's doll, Danny O'Day.
There are two different versions of the "Mary Shaw" poem. In the movie the poem goes, "Beware the stare of Mary Shaw. She had no children only dolls. And if you see her in your dreams, be sure you never, ever scream." On the trailer it was, "Beware the stare of Mary Shaw. She had no children, only dolls. And if you see her do not scream, she'll rip your tongue out at the seam."
Leigh Whannell was so unhappy with the finished product, due to studio interference, that he decided to write all future scripts on spec, as opposed to pitching an idea to a studio and then being paid to write the screenplay, as was the case with this film.
The trivia items below may give away important plot points.
Not counting any previous victims of Mary Shaw, the body count seen in the film is six: Jamie Ashen, Det. Lipton, Henry Walker, Edward Ashen, Michael Ashen, and Lisa Ashen. Eight, if you count Lisa's unborn baby and Mary Shaw herself. Ten, if you count Bos, the gatekeeper, and Ella Ashen (corpses only shown in a deleted scene).
As every single main character dies in this film, the only survivor of this movie is supporting character Marion Walker, she never even encounters Mary Shaw at any point. She does, however, come close to seeing her when she finds Henry's corpse, having been killed by Mary Shaw.