The Three Coffins by John Dickson Carr
p. 67 – “It’s much easier to believe that a man walked on snow without leaving a footprint than to believe he knew precisely when he would have it to walk on.”
When the threatening stranger came to visit Professor Grimaud, Grimaud’s secretary saw the professor open the door of his room and saw the visitor go in. Soon afterward, several people heard the sound of a shot from inside Grimaud’s room. When they broke down the locked door, they found the professor shot and dying – but nobody else was in the room!
The visitor couldn’t have climbed up the narrow chimney. He didn’t leave by the door, because Grimaud’s secretary was watching. He didn’t go out the window, because the sill was covered by several inches of fresh snow.
Then again, just how did he get into the house – since the front steps were also covered by unmarked snow? It must be some sort of trick, with an alibi timed to line up with the snow.
Oh. Wait a minute. It’s one thing to work out a detailed alibi and time it to the second. But how can anybody plan the timing of a snowstorm?