The house is supposed to have been built in the 1850s, and the interiors are typical Victorian era mansion sets, but exteriors are of a house designed in the 1920s by Frank Lloyd Wright which still looked very modern when this film was made more than 20 years later.
Lance locks his door from the inside with a deadbolt after the Doctor calls everybody to the meeting, and leaves through Nora's door. When they return from the meeting he opens his own, unlocked, door from the outside.
In the opening scene, when Frederick Loren introduces each of the party guests, the back window of each person's funeral car is blacked out with a dark cloth. But in all the long shots showing the procession of cars driving slowly along the road, the back windows of all the cars are clearly transparent, with nothing blocking the view through the glass.
The skeleton's dissolved by the acid should not be held together in one piece the way the are seen, as the acid should have melted the tendons and joints that hold bones together, thus resulting in piles of loose bones.
Frederick Loren announces at the start of the challenge that if any of the contestants die, the deceased's heirs will get his/her $10,000. Later he says that if any of the five contestants die, the total $50,000 will be split evenly among the survivors. However, the initial statement is spoken directly to the film's audience, is not part of the actual plot line, and therefore does not really contradict the terms subsequently described to the genuine characters.
The rope that comes floating through the window and wraps around Nora's feet and then unwraps itself and goes out the window again. In real life, how would you do all that? You can't reverse film in real life, and if there were a string attached, wouldn't Nora see it or feel it?
When using candles in the rooms in the basement, the candlelight is obviously a spotlight as the shadows of the candle and silhouette of the person are displayed on the wall, when all there should be is candlelight.
Mr. Loren shakes an unopened bottle of "champagne" and points it menacingly at his wife, yet when the bottle is opened there is no effervescent "fountain" nor bubbles visible in their glasses when poured.
After Trent and Lance lower the hanging body of Annabelle and are joined by Loren, Trent states that she hanged herself. However, just a few minutes later when all but Nora are gathered in the parlor, Trent explains how it was clearly a physical impossibility for Annabelle to have done herself in.
Loren drops the skeleton back into the acid from which it had emerged unscathed. Some have interpreted this act as destroying the evidence of his crime and the two being self-contradictory. However, there is no hint of motive for Loren's returning it to the vat and no bubbling effect to indicate any dissolution of the skeleton. Furthermore, Watson Pritchard had not only previously stated that the acid leaves bones behind, but he had demonstrated it by tossing in the body of a dead rat.
After the guests have locked themselves in their rooms, Nora emerges, goes to the end the hall and discovers Annabelle swinging from a noose. Nora screams. She backs into a corner and is confronted with a monster hand and screams again. Next scene, the doctor knocks on Mr. Loren's door and asks him if he heard anything strange, to which Mr. Loren replies that he thought he heard footsteps. How did no one hear Nora screaming?
If the caretakers are real people, how can they stand in the doorway without saying a word and suddenly disappear? Also, how does the wife float in mid-air, glide around the cellar and not be seen by Lance when she passes through the doorway to the next room where he is waiting? She doesn't react to a bloodcurdling scream right next to her - is she deaf as well as blind? If she is blind, how does she head straight for and cleanly through that doorway?
It is established that no one can get out of the house due to the locked metal door and bars on all the windows, yet Annabelle somehow gets outside so she can pretend to be a ghost outside Nora's window.
Annabelle complains that her husband invited all the guests, and they're strangers. But the psychiatrist is secretly her lover and will help her try to murder him during the party. So doesn't she find it odd that her husband invited him? She obviously didn't suggest that he do so.