In most mystery novels, the author drops enough clues and hints that a really attentive reader might guess the outcome. Not so in The Moonstone. The fact that Franklin Blake, the cousin who is most engaged in trying to solve the mystery, is actually the thief is surprising enough. But what reader would have guessed that he took the diamond unconsciously while under the influence of opium that Dr. Candy and Godfrey Ablewhite had slipped into his bedtime brandy?
Some readers feel betrayed by the ending of The Moonstone: they have faithfully read all the narratives, hoping to be able to solve the mystery based on the evidence presented by Betteredge, Miss Clack, Mr. Bruff, and the others. But nothing prepares them to guess that Blake took it while on opium. No one sees it coming.
That raises a really interesting question about reading. It's our expectation of the mystery genre that the author will give the reader the tools required to solve the mystery. When those expectations aren't met, some readers get irritated. But when you come right down to it, why is the author obliged to follow some unwritten generic conventions? What are the obligations of the author to the reader? Why do unexpected twists sometimes annoy us?