Mysteries should take advantage of their premise. If the story takes place in a unique setting, then the solution should involve a trick that's only possible in that particular location. If the hook of the crime is a unique or bizarre state of the crime scene, then there should be a compelling reason the crime scene was in that state.
In The Eight of Swords, the culprit leaves an Eight of Swords at the crime scene. Quite easy to understand, no? That tarot card is one of the main draws of the book; after all, it's what the novel is titled after! And yet... there's no compelling reason for that card to be there. Heck, I have absolutely no idea why the culprit left the card there, other than to provide a clue to Dr. Fell to use to deduce their identity.
