I find that discussing an idea out loud is often the way to kill it stone dead.
Yes, it's rather funny, really, that next to no-one realized the snake that Harry set free in Philosopher's Stone turned out to be Voldemort's final Horcrux, Nagini
OH NO YOU DON’T, LADDIE!” Harry spun around. Professor Moody was limping down the marble staircase. His wand was out and it was pointing right at a pure white ferret, which was shivering on the stone-flagged floor, exactly where Malfoy had been standing.
Stone dead," said Howard, as though there were degrees of deadness, and the kind that Barry Fairbrother had contracted was particularly sordid.