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.

Clue (1985)
Plot summary
Directed by: Jonathan Lynn
Starring: Christopher Lloyd, Tim Curry, Michael McKean, Lesley Ann Warren, Madeline Kahn, Eileen Brennan, Martin Mull
The six people from the game "Clue" (Colonel Mustard, Mrs White, Mrs Peacock, Mr Green, Miss Scarlett, Professor Plum) are invited to dinner at a big house, and are met by the butler, Wadsworth.
Wadsworth explains to the guests that they are individually being blackmailed by Mr Boddy, who is also present. Mr Boddy then hands out weapons (the 6 weapons from the game: candlestick, rope, lead pipe, wrench, gun, dagger), and invites the guests to kill Wadsworth to avoid being exposed.
Instead, someone kills Mr Boddy. During the evening, various other people (the cook, the maid, and several vistors) are mysteriously killed one at a time.
It turns out that all the other victims were Mr Boddy's accomplices, and Wadsworth had gathered them all together to give evidence against Mr Boddy's blackmailing. Wadsworth explains how each murder was done. Who did each murder is another question - the movie has three different endings, each with different killers.
Wadsworth: I can explain everything...
Cop: You don't have to.
Wadsworth: I don't?
Cop: No, there's nothing illegal about any of this.
Wadsworth: Are you sure?
Cop: Of course, this is America.
Wadsworth: I see...
Cop: It's a free country, don't you know that?
Wadsworth: I didn't know it was THAT free.





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.