I'm reading a first-person sort-of detective novel (that is, the detective is an amateur), and I'm about in chapter 10, and I figured out who the murderer is. How? Not by assembling the clues or anything. But a secondary character (who seems benevolent) is mentioned twice in the same paragraph by another character. He's a forensic tech, and they're discussing who goes to a crime scene, and the ex-cop says, "Then the forensic tech does this. blah blah blah Then of course there's the forensic tech's work."
And I said to myself, "The forensic tech did it."
I yield to no one in my admiration for crime writers who have to somehow make it clear-in-retrospect that this man did the murder, and yet not let the reader guess that till the end. And it's just getting harder with readers like me, who guess not based on the meticulously planned clues, but on trivial little meta things in the narrative. ("If Grandma's pearls are mentioned three times, then that means Grandma must have killed him!" "Oops. The only person being nice to the detective is that banker, so he must be the murderer! The murderer is always the nice person.")
So... first, have you ever guessed the murderer in a mystery? And how did you do it? What triggered the realization?
And if you've written a mystery of some sort, how did you conceal and yet reveal?
And if you don't write a mystery, let's have some sympathy for those who do. :)