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.
Continuity mistake: When they reach the study with the dead cook's body, they stop before entering and drop her half-way into the room. They proceed to look for Mr. Boddy in the room and file into the Study one at a time. In the same camera angle you see everyone entering the study including Miss Scarlet, then Yvette, then Mrs. White. Then the camera angle changes and in comes Scarlet, White and then Yvette.
Character mistake: During the conversation in the study about who is blackmailing who, Mrs. White mentions two of her five husbands: An illusionist who disappeared and never reappeared and a nuclear physicist who was found dead at home. During the last ending, Mrs. White says "So this has nothing to do with the disappearance of my nuclear physicist husband or Colonel Mustard's work on the new top secret fusion bomb?". However, during the conversation in the study, Mrs. White says her illusionist husband had disappeared and her nuclear physicist husband was found dead at home. So the correct term would be "So this has nothing to do with the disappearance of my ILLUSIONIST husband or Colonel Mustard's work on the new top secret fusion bomb?". (00:21:05 - 01:27:35)
Other mistake: During the entire movie, the candles on the dinner table are lit and yet never seem to get any shorter as the movie progresses.
Revealing mistake: A visible editing flaw occurs near the beginning, after they return from the Billiard Room with Yvette. While Mrs. White is saying her line to Wadsworth, "I think you had better explain," she starts to unbutton her overcoat. Next is a wide shot of the room that is supposed to be in real time, but Mrs. White's overcoat has already been removed, and she is getting ready to lay it on a chair. Everyone is paused on their marks as if the film was spliced at the exact moment the director yelled, "Action" and no one had moved yet. (00:31:12)
Continuity mistake: When Professor Plum turns over Mr. Boddy to see where he's apparently been shot, his eyes are closed. In a flashback sequence later in one of the endings (where Miss Scarlet and Yvette are revealed to be the killers), after Plum turns Mr. Boddy over, his eyes are open instead.
Audio problem: In the scene in the study where the cast gangs up on Mr. Boddy (when he and Colonel Mustard get into a fight), right when Mrs. White knees him in the groin you hear a lot of commotion from the other guests in the background. Included in this is Yvette saying "Oh no it's." (we know it is Yvette because of the distinct french accent) when in fact, she is in the billiard room recording the conversation.
Revealing mistake: Near the end, in the first ending, when Yvette is killing Mr. Boddy, you'll see that she stopped the candlestick above his head when she hit him with it.
Revealing mistake: When Yvette shoots the chandelier the reflections of the lights can be seen on the hall floor in the high angle shots. the lights stay still when the chandelier is spinning and falling.