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.
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).
Revealing mistake: When the cook's body is lowered to the kitchen floor, you can see her move her outstretched arm closer to her head.
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.
Visible crew/equipment: After opening the doors to the lounge to let Col. Mustard and Miss Scarlet out Mrs. White says "How did you get in the doors were locked?" After this line, for the remainder of the scene, on the left side of the wall beside the chair the reflection of a crew member and the boom mic pole can be seen moving around.
Revealing mistake: In the hallway, the camera pulls back from the dining room towards the front door. The camera pulls back too far, and on the floor masking tape and a different type of wood can be seen, indicating where the set stops.