Author's Note: A brief and insane Fidelius exploit I've never seen. Very silly.
"They were often joined by other Order members for dinner now, because the Burrow had replaced number twelve, Grimmauld Place as the headquarters. Mr. Weasley had explained that after the death of Dumbledore, their Secret-Keeper, each of the people to whom Dumbledore had confided Grimmauld Place's location had become a Secret-Keeper in turn.
"And as there are around twenty of us, that greatly dilutes the power of the Fidelius Charm. Twenty times as many opportunities for the Death Eaters to get the secret out of somebody. We can't expect it to hold much longer.""
-Deathly Hallows, Chapter 6
"Lucius, I have chosen you as my Secret-Keeper for the Fidelius Charm."
"Y-you have? I am honored, my Lord."
"The Secret is that my schoolboy diary is buried beneath the dungeons of your manor. Fidelius."
"I'm... I'm not sure why you needed to tell me this, my Lord, but I am honored beyond words that you could summon up the absolute trust required to use me as your Secret-Keep-"
"Oh, I'm absolutely sure you won't betray the Secret, Lucius. Avada Kedavra."
As the body hit the floor, Lord Voldemort smiled. He was going through Death Eaters at a painfully high rate, but it was absolutely worth it. He had already hidden the Secrets of his identity as the half-blood Riddle, the locations of three out of the five Horcruxes he had already made, and his creation of Horcruxes in the first place. Since the death of a Secret-Keeper caused all those who knew the Secret to become Secret-Keepers in turn, that left him as the sole Secret-Keeper for every one of those Secrets save his identity as Riddle, and Dumbledore would have to manually disseminate that little piece of inconvenient to each person whom he wanted to tell in turn. Slughorn knew he had thought of creating Horcruxes... but, strictly speaking, Slughorn did not know that he had, and thus he too was stricken by the Fidelius.
Lord Voldemort would dispose of him when he had the opportunity, but that was just to tie up loose ends. After all, Lord Voldemort had already entrusted his deepest, most dangerous secrets to the one being he trusted in all of existence - himself.
With a spring in his step, he strode off to determine who should have the honor of dying for the Secret of the location of Hufflepuff's Cup. He'd thought of Bellatrix, but she was too useful. Perhaps that little Half-Blood, the Prince scion? A pity, as he reminded Lord Voldemort in some ways of his younger self... but sentimentality was a good way to invite daggers in the back. Only an imbecile would suffer a younger Tom Riddle to live, knowing his true nature.
Severus it was. Lord Voldemort gave a pleased sigh. What a delight it was to know the secret of his immortality was truly invulnerable.
Author's Note: This really can't be right. Mutual Fidelius was bad enough. Autofidelius is an abomination before logic and nature. There must be some implicit rule that the direct beneficiary of a Fidelius Charm can't become a Secret-Keeper. There must be...