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.
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
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.
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: 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.