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).
xx:xx:xxJames R P
Continuity mistake: After Mrs Peacock turns the lights back on in the study, she drops the dagger where she is standing, by the door. A few moments later when she is screaming about the 'poisoned' brandy, the dagger is no longer there. That area of the floor can be viewed as Mr Green is calming her down and the dagger is nowhere to be seen. This is explained in one ending, as Yvette crept in and stole the dagger in between time. However using the real ending (i.e. the one where the title card reads 'Here's how it REALLY happened') then this is a massive goof, particularly as Mrs Peacock is supposed to kill the cook with the dagger in a few moments time.
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.
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.
Continuity mistake: When Wadsworth is doing the re-enactment of the murderer killing Mr. Boddy with the candle stick, he pretends to hit Mr. Green. After he says "Will you stop that?!", Wadsworth grabs him with one hand. But when he throws Mr. Green into the toilet, he is holding him with two hands.