Other mistake: In one of the first scenes in the study, Mr. Boddy hands each guest a weapon and then proceeds to turn out the lights. The room is thrown into complete darkness. Unfortunately, there is a roaring fire in the fireplace that would have illuminated the room enough for everyone to see what was going on.
Deliberate mistake: The same footage of Ms. Scarlet screaming when the cook falls out of the freezer is used when Wadsworth falls out of the freezer. (Note the cigarette.)
Visible crew/equipment: Towards the end of the film, when the butler is explaining 'whodunit', as he stands at the front door, quite clearly at the top of the screen a microphone can be seen following him back and forth.
Revealing mistake: After they find Mr. Boddy's body in the bathroom, Wadsworth starts shouting. A candlestick then falls and hits him on the head. Look at the candlestick when it falls: you can tell it was pulled by a string or something.
Factual error: When the evangelist first enters the mansion, Mrs. Peacock calls him a beatnik. However, the movie is set in 1954, and the word "beatnik" was not coined until 1958.
Audio problem: When they are putting the Cook's body on the sofa, Wadsworth walks by in the background and says "Careful, don't get blood on the sofa" however his mouth doesn't move.
Other mistake: When a gloved hand tosses an envelope's contents into the fire, most of the papers miss the fire completely.
Other mistake: In the study, when Wadsworth says that Mr. Boddy is the one that's blackmailing everyone, Colonel Mustard and Mr. Boddy get in a fight. Mr. Boddy pokes Colonel Mustard in the eyes. You can tell that he stops his fingers right in front of Mustard's eyes. Less than a second later, Colonel Mustard falls over.
Continuity mistake: When everyone is in the Library, you can see the matches in a cup on the left of the fireplace. When Professor Plum says, "Wait a minute, suppose that one of us is the murderer," you can also see that the matches are in a "V" shape. When Wadsworth walks towards them to pull the matches out, most of them are all leaning to the left side of the cup. As the movie goes on, when the cop gets locked up in the Library for the second time, the same amount of matches are in the holding cup as if Wadsworth did not pull them out and they are in a straight line. (Third time code 1:02:40).
Continuity mistake: When Wadsworth goes in to talk to the cook at the start of the movie there is a boiling pot of water on the stove. When she turns to him with the knife it is gone, then when he turns to leave the kitchen it is there again.
Other mistake: When Mrs. White enters the house, she takes off a black and white cape so she now is wearing a light coat over her dress. There is a point at which she is about to take off the coat, and with the camera angle change, she has it in her hand.