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: When a gloved hand tosses an envelope's contents into the fire, most of the papers miss the fire completely.
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.