Revealing mistake: Right after Nell sends Hugh to hell, Dr. Marrow and Theo come out of hiding behind a pillar. As Marrow puts his hand on the pillar, it squishes in as if it was made of foam rubber, instead of stone. (01:39:50)
Continuity mistake: When Nell arrives at Hill House, after Mr. Dudley opens the gate and she drives through, between shots the amount of brown leaves on the ground around the gates increases.
Continuity mistake: While they're all having their first dinner at Hill House, everyone has a little red wine. The wine levels in Lili Taylor's glass go up and down between different shots. (00:24:35)
Continuity mistake: As Nell pokes around in the furnace under the fireplace with her iron poker, the bones and ash in it tend to move around, changing their positions many times between shots.
Audio problem: When Nell is helping Theo carry her bags to her room, Theo turns to Nell and says "well, everyone calls me Nell, don't you love it here?" Immediately, Theo then says "I love it." But her lips don't move, she's just smiling. (00:15:35)
Continuity mistake: When Nell finds the picture album of Hugh Crain and his wife it passes around 3 or 4 pages. But when Nell grabs it, it's about 10 or 20.
Visible crew/equipment: At the end of the film, when the Dudley's arrive, Mr. Dudley opens his door, and in the far right of the door window, a crewmembers head is reflected.
Continuity mistake: When Theo and Nell are talking in Theo's bedroom, in one shot Theo removes her black cardigan. In the next shot, you can see in the mirror, that she removes it again.
Continuity mistake: During dinner, Theo starts pouring a glass of wine for Mary. In one shot she has poured about a quarter of the glass full. But in the nest shot the glass is nearly full. (00:23:50)
Continuity mistake: When Nell is in the study, the photo album of Crane opens. It then nearly falls so she grabs it. As she grabs it you can see the page has a picture on it, of two people's face very close up. But in the next closeup of the same page, the picture has changed and it is now Crane standing alone in the centre of the picture. (01:02:50)
Continuity mistake: When Nell is searching for Dr. Marrow, and she stops his work on the table, the tape recorded on the table changes position between shots. (01:00:15)
Continuity mistake: In the scene in which the girls enter the playroom with the revolving floor, you see them, in a shot from the back, step onto the moving floor. In the next shot, from the top, they are back in the entry and step onto the moving floor a second time. (00:21:05)
Continuity mistake: When Nell runs to the Circular Room the second time, she is looking at one of the mirrors. She sees herself pregnant. If you look close, you can see her reaction in the mirror. She brings her hand over her mouth. In the next shot, she does the same thing again.
Continuity mistake: At the first afternoon, Nell is filling out some tests in the Great Hall. We see her put down the same paper twice, one from the ground shot; the other from overhead.
Continuity mistake: When Nell walks to the bookcase, where she was led by the bloody footprints, the vase of peacock feathers on the desk in front of her, change positions between shots.
Other mistake: Nell's hair is considerably lighter in the scene where it starts moving by itself - clearly CGI.
Continuity mistake: When Nell opens the ledger, it shows the first page. Two people are crossed out in the ledger. In between the rows which have been crossed out, there are ten rows of recorded people, which aren't crossed out. Nell then puts the ledger on her lap, and is still on the same page. Yet when it shows the page again, there are now only five rows in between the two crossed out ones. (00:55:35)
Continuity mistake: The confetti-like things that are on Theo's hair in the final showdown, disappear in subsequent shots.
Factual error: The Crane mansion is supposed to be set above the village of Concord as listed in the ledgers, however, Nell answers an ad directing her to the Berkshires which are nowhere near Concord.
Answer: Another possibility is the spirits of the children. As long as Hugh Crain was inside the house, the children were trapped inside too. They could have called Nell because they needed her help in being set free.