Continuity mistake: When Tony throws the 100 pounds down-payment across the room to Swan, watch the chair in which it lands. It has a low back one in one shot, then a high back (probably to provide a bigger target for the throw), then a low back again.
Dial M for Murder (1954)
Ending / spoiler
Directed by: Alfred Hitchcock
Starring: Ray Milland, Grace Kelly, Robert Cummings, John Williams
Halliday, Margot & Chief Insp. Hubbard all discover that it was Tony who set up Margot's attempted murder. They find out because Tony grabbed Swann's key and put that key in his wife's purse and the other key (his wife's key) was put back under the staircase cover right after Swann opened the door with it (Tony assumed that he was getting his wife's key from Swann's pocket, because Swann was supposed to put the key back under the staircase cover after he killed Margot, but he got Swann's own key instead). When Tony opens the door with the key that Swann put back under the staircase cover, he sees Halliday, Margot, Insp. Hubbard & Det. Williams and he knows that he has been caught.
Big Evil
More mistakes in Dial M for Murder
Margot Mary Wendice: How long have you known this?
Chief Insp. Hubbard: Did you suspect it yourself?
Margot Mary Wendice: No, never. And yet... What's the matter with me, Mark? I don't seem able to feel anything.
Trivia: Alfred Hitchcock wanted the film to look as natural as possible, with the camera at eye level with the actors. Since movie cameras were large (especially 3D cameras), Hitch had a small trench built into the soundstage floor, so that the camera lens was, roughly, at eye level.