Revealing mistake: When the house has been locked down at the beginning of the film no moonlight could come in, however when the guests are being shown the coffin with the guns in there is a shaft of bright moonlight shining on to it.
House on Haunted Hill (1999)
Plot summary
Directed by: William Malone
Starring: Famke Janssen, Geoffrey Rush, Taye Diggs, Peter Gallagher
Six guests are invited to Evelyn Stockard Price's birthday...Steven Price, Jennifer Janzen, Eddie Baker, Donald Blackburn, Melissa Marr, and Evelyn herself. The only problem? Those weren't the people on Price's list. When the six guests are trapped in the house with Watson Pritchett, the man Price rented the house from, the spirits within the house want revenge for something that happened long ago.
More mistakes in House on Haunted Hill
Peter Graves: Dr. Richard Benjamin Vannacutt. He out-butchered Bundy, made Manson look meek.
Trivia: The darkness that emerges and begins to kill the guests was actually created when the animators took film of dancing naked women and mirrored the image over and over.
Question: Who exactly is the person that Melissa sees behind her before it attacks her?
Answer: Just a generic ghost/monster as it has no face.
Answer: The ghost that kills Melissa is the patient Vannacutt was dissecting. When Melissa initially sees them through her camera, what she's really seeing is the ghosts re-living their last moments.