Plot hole: In the end, when Charlie is advancing on Emily while turning his light on and off, the final time he turns it on, Kathrine is in front of Emily. How could she get there so fast without being heard? There's at least 4-5 inches of water that would make sneaking very hard.
Continuity mistake: When David is writing about Emily needing more time, etc. you see three lines in the close-up of him writing. When he picks up his hand and notices the spot on his palm there are 5 lines of writing on the paper. (00:17:45)
Continuity mistake: When Emily is on the merry-go-round, she has permanent front teeth, but later in the film she still has baby teeth.
Continuity mistake: When David (in fact, he was Charlie that time) was looking for Emily in her room, he checked if she was under either of the two beds. After he bent down, he was checking the bed on his right side, but when he was shown from another camera angle, he was checking the bed on his left side, before he lifted his head again. (00:50:00)
Revealing mistake: Katherine shoots Charlie twice to kill him at the end of the movie. Yet if you look at the chambers on the side of the gun, you can see all of them are empty.
Answer: He keeps singing "mockingbird" over and over, but it is more of a mumbling by this point. Turn on the subtitles and you can see this.
EMTurbo