Revealing mistake: When the cook's body is lowered to the kitchen floor, you can see her move her outstretched arm closer to her head.
Revealing mistake: After they find Mr. Boddy's body in the bathroom, Wadsworth starts shouting. A candlestick then falls and hits him on the head. Look at the candlestick when it falls: you can tell it was pulled by a string or something.
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.
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)
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.
Revealing mistake: When the 4 people upstairs, Yvette, Mr Green, the butler, and Mrs White all come running downstairs because Col Mustard and Mrs Scarlet are yelling at the locked door they collide at the top of the stairs. When they all hit they're stunt men and women. (00:54:15)
Revealing mistake: Near the end when Miss Scarlett points the pistol in Col. Mustard's face, over his response to her revealing everyone's secrets, you can hear the trigger click twice, like she'd accidentally shaken the grip, or an accidental pause or prop failure.
Revealing mistake: Just before the chandelier falls, Col. Mustard quite visibly flinches, even though, realistically, he would have no way of knowing that it was about to crash behind him.
Answer: As Wadsworth explained to everybody, when he saw the motorist at the door, he didn't throw the key away but put it back in his coat pocket. While everyone's attention was on the motorist, Col. Mustard took the key from Wadsworth pocket and replaced it with a different key.