Continuity mistake: When Annabelle gives Frederick her gun, he places it in the mini-coffin and closes the coffin. However, in one of the next shots, we see that all coffins are open.
Suggested correction: It's not "in one of the next shots" it's actually many shots later, so anyone could have touched that mini coffin for any reason during the time off-screen. After Frederick closes the top of the mini coffin, it cuts to the scene upstairs with Nora and the missing severed head. Then a considerable period of time has passed until we see the mini coffins again, when Lance walks into that room and sees Pritchard with the knife.
Plot hole: When Mr. Loren introduces his wife to everyone, he tells her, "You know Watson Pritchard of course." That can't be because at the beginning of the movie the characters are asking if any of them know Frederick Loren. No one personally knows him including Watson Pritchard. He says, "I don't know him, all the details about renting the house were done by mail." If that's the case, it's impossible for Annabelle to know Pritchard since Frederick invited all the guests without his wife's approval. (00:34:05)
Suggested correction: By no means a correction, but I always took this line as a reference to the fact that in the film's universe the case of the house being haunted, multiple murders on site, etc., is well known, and thus Watson Pritchard by default is already infamous/known of by the wife of Frederick Loren, rather than a reference to them personally knowing each other before the events shown on screen.
Other mistake: Once Dr. Trent enters the bedroom where Mrs. Loren is lying down "dead", they begin talking. Trent says "At first I couldn't get Nora to protect herself with a gun." There is no scene in the movie where he tries to convince her to use the gun, Lance is responsible for that. (01:02:25)
Suggested correction: After Annabelle Loren is found hanging under suspicious circumstances, both Lance Schroeder and David Trent advise all the other guests to stay in their rooms with their guns and shoot any intruders.
Just before he leaves for the meeting, Lance tells Nora, "If you have to, you use it." Nora was not at the meeting to hear what was said, so ultimately, it was Lance who got Nora to protect herself with the gun.
There's an important mistake in your version of Dr. Trent's quote. What Trent actually says is, "At first I couldn't get Nora to want to protect herself with a gun." He's not saying that he personally advised her to protect herself; he's saying that he, through his various scary tricks, had failed to scare Nora into arming herself. Trent's immediate next lines are: "But after you appeared at the window, everything began to work just as we planned. You were wonderful, just the touch that finally drove her into complete hysteria." We know that David Trent and Annabelle Loren were behind a number of staged scares (such as the witch in the basement, the attack on Lance, Annabelle's hanging, etc) intended to push Nora toward hysteria, setting her up, of course, to kill Mr. Loren "by accident," thereby committing the perfect crime.
Factual error: At the very beginning, while Frederick is telling us about the party, he says, "There will be food and drink and ghosts etc..." Throughout the movie, there isn't a single morsel of food ever seen or offered.
Suggested correction: Around thirteen minutes into the film, when Frederick starts to pour drinks for his guests, you can see a table with food on it behind them. Appears to be sort-of of a small, buffet style selection of hors d'oeuvres, albeit it's out of focus in the background. (It's very easy to miss.)
