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.
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.