It began with a mistake.
Actually, it began with several mistakes, and no one involved ever managed to declare himself responsible without someone else being able to point out an earlier mistake.
Chances were that it really began with one Merope Gaunt brewing a love potion. Except that even before that, her great-great-great-great-great-great-grandfather had decided no one without a drop of Slytherin blood and with any Muggle blood whatsoever was to marry his descendants, and as such witches and wizards were rather rare to begin with, well...
But even that was based in the error of the sons of Slytherin, which had its foundation in...
As Lily Potter nee Evans once declared when the Blame Claim started up, "Just trace it back to Eve and the apple and be done with it before you wake Harry again."
The mistake in question, the one that mattered, occurred on October 29, 1981 at 9 pm, at an Unplottable location not too far outside the wider London area.
It was the result of an earlier mistake involving a rather embarrassing confluence of events no one could have foreseen, leading to one paranoid Dark Lord's anger as well as a growing thought that he might have just located a potential traitor.
The end of a mistake and the beginning of another: one Death Eater named Severus Snape lying in lingering agony on a stone floor, absolutely certain that the Dark Lord formerly known as Tom Marvolo Riddle knew just what he'd been up to the past year and a half.
After all, why else would he have been called to said Dark Lord's side, questioned under the Cruciatus Curse repeatedly, and then left alone on a school night when he would be expected back at Hogwarts unless that Dark Lord believed him to be a double agent?
Particularly when his service to the Dark Lord - spying on the headmaster - specifically required him to get and keep that job.
A chill settled on him. If the Dark Lord wanted proof he was a double agent, well, there was plenty of evidence to be found.
If he wanted to live, it was time to be a good little Slytherin and find a way the hell out now.
The wards on the door were easy enough to overcome even wandless - which should have been a sign - and Death Eaters were so used to both carrying their wands out of sight and having the living daylights cruciated out of them that Severus' departing in that condition didn't seem out of place.
The Dark Lord hadn't said he couldn't leave, and if he'd decided the young potion master was a loyal spy after all, well, who was going to question it?
What Severus did not know was that evidence of his loyalty was even now being provided and that in a few moments his wand would have been returned with no apology given. He would be even more trusted than before, and in a few days the 'failure' for which he was tortured would have ceased to matter entirely in Wizarding history.
What he did know was that he overheard a whispered comment just as he raised his left arm to pass the wards on the front door, stating that wasn't it odd Snape was allowed to be there at the same time their lord was meeting with that dirty little rat?
He didn't miss a step, even as his mind remembered that some of the worst attacks of his NEWTs prep years had been prefaced by a castle rat nosing around and then disappearing around a corner... only to have James Potter's gang appear less than ten minutes later.
Often much less.
It had been as reliable an indicator of their presence as Filch's cat was of future detentions.
It was no secret in the Order that there was a suspicion of a spy in their midst, and in fact Severus had been asked by Headmaster Dumbledore to keep his ears open about it.
He needed to get out NOW.
The Knight Bus, each and every bump, pothole, speed bump, and too-fast turn (with Ernie's driving, they were all too fast) sending shocks through him.
Severus had lost count of the curses thrown at him, and now he was beginning to wonder if he'd set some record for quick mobility. The conductor had nearly insisted that he go to St. Mungo's instead of Hogsmeade, based on the look in his eyes alone.
But no, professors had privileges, one of which was no-questions-asked prepaid transport to and from the school area when needed, as well as access to Madame Poppy Pomfrey when injured. And the sooner Severus could get to Dumbledore's office...
No wand. A sickle and three knuts. Two linty lemon drops Headmaster Dumbledore had shoved on him a lifetime ago that morning. One rattled nervous system. No endurance left. Heart pounding. Mark burning, but not even the Dark Lord was strong enough to risk an attack on the Knight Bus.
Enough of his infamous sarcastic tongue left that there was no question at all about who he was.
He barely stumbled into Aberforth's bar and managed to fall through the fireplace when Aberforth finished setting up the temporary Floo connection to Albus' office.
The headmaster was already on his feet.
"Cover... blown..." he managed to gasp to the older wizard before he completely passed out.
It was 1 am, October 30, 1981.