Revealing mistake: When the 4 people upstairs, Yvette, Mr Green, the butler, and Mrs White all come running downstairs because Col Mustard and Mrs Scarlet are yelling at the locked door they collide at the top of the stairs. When they all hit they're stunt men and women. (00:54:15)
Continuity mistake: When Yvette serves Mrs. Peacock she says shark fin soup but then on 2 occasions later when Wadsworth is going back over how they did it he refers to the soup as a monkey brain, popular in Cantonese cuisine.
Suggested correction: There's actually 2 separate dishes the shark fin soup and then the monkey brains which is that white fluffy stuff they eat after the soup.
Continuity mistake: When the 1st chandelier fell, we see Peacock and White are standing near the front door. Scarlet then says "look!" Bringing everyone to look at the dead motorist in the study. Peacock and White are suddenly teleported to the study door behind Scarlet, even though they were quite far away with even a couple steps to climb down.
Continuity mistake: In the "C" ending, the one in which everyone is partially guilty, when Mr Green lets the cops in and they run into the hall, the shot from behind the other guests shows them put their hands up as the cops surround them. When the camera angle then shifts to show them from the front, their hands are down.
Plot hole: When Miss Scarlet tells the other guests, they can pay her in government information, she forgets Mrs White is the only to have no connection to the government and thus can't pay her that way, nor does she have any more money like the others.
Revealing mistake: Near the end when Miss Scarlett points the pistol in Col. Mustard's face, over his response to her revealing everyone's secrets, you can hear the trigger click twice, like she'd accidentally shaken the grip, or an accidental pause or prop failure.
Continuity mistake: When Professor Plum and Miss Scarlett enter the porch, the camera closes in on his hand touching her bum. But the following shot shows his arm at the center of her backside as she 'edges' away from him.
Continuity mistake: In the beginning when Col Mustard shows up, the butler shows him to the library, you can see that door to the library is open. When the scene shifts Wadsworth is opening the door. (00:05:30)
Other mistake: Why on earth would six people from Washington, D.C. be summoned, at night, to a house in New England (as we learn from the opening)? That's a drive of between six and twelve hours depending on where in "New England" it was. Yet none of them remark on the very long drive, or mention staying in hotels, or talk about luggage, or have luggage, or complain about having to stay overnight.
Suggested correction: They are being summoned concerning the blackmail that is happening to them. They have no choice but to go. I don't think any of them were planning on staying the night at Hill House. They were there for dinner and the news about the blackmail. If they have luggage, it is most likely still in their cars.