The Room of Requirement was not perfect. It was fairly close, having been constructed by Merlin himself, with Hogwarts later built around it and adapted to suit the magic it radiated. It was said to be one of the Eleven Wonders of the Magical World, but no one is quite sure as the last person to know more than three of them, Manal Haddad of the 14th century, died without passing her knowledge on. It is a room unbounded by space or time, unique in the universe as far as the ministerial arithmancers can tell. It is capable of creating anything conceived by its occupants. But still the Room is imperfect. It bears two major faults.

Firstly, that the Room has very little power to check itself, or prevent its own abuse. It is very nearly a miracle that some nefarious or simple-minded student hasn't wandered in and asked for a creative new plague, or a second big bang. Nearly miraculous, because it was not the work of a divine being, but of a careful network of ghosts monitoring knowledge of the Room, and requesting obliviation for those students with ill intent or unwatched tongue. It was a fine line they walked, but often enough students dismissed the full potential of the Room, and kept it as a private recreational space. Most never found the Room to transform beyond a cozy lounge, for privacy and hominess were so often the chief desires in a crowded boarding school. Most headmasters never need be informed of the Room. In recent years, relative for a ghost of course, they had discovered that they could ask the Room for help when even the headmaster proved untrustworthy, though this method had been generally avoided after Peeves accidentally revivified himself. He'd never really been the same since the odd suicide that began his second unliving. But I digress. Suffice it to say that the Room had been kept relatively far enough from the Hogwarts' rumor mill that no one, not even the ghosts, ever discovered the second problem of the Room of Requirement,

The Room of Requirement seldom contained multiple individuals within its walls. Even when it did, the groups that came usually entered with a common goal for the Room in mind, be it a study space, a private kitchen, a less drafty shack whose shrieks were far more pleasurable. Once even a spelunking cave, designed to be the perfect depth to push older brothers into, had only the unfortunate first year possessed enough arm strength to shove them. Scarcely three dozen groups of more than five individuals had entered the Room, for all its history, and always with a consistent agenda. So it came to be 1996, with no one knowing or having considered what happened when the Rooms occupants disagreed.

And disagree they did. The moment the distinguished, or, at least, thoroughly distinguishable, undersecretary's bombardia rocked the doors to DA headquarters, all common primary desires, to practice defense safely and secretly, vanished. In that absence, a plethora of secondary desires emerged into primacy. Desires from without the Room to be within and within the Room to be without, but not without because there seemed to be an incensed pink toad seething pleasantly in the corridor. Desires for this to not be happening, to be home with parents, to be safe, and not at war, and not the chosen one, and as far from the Umbitch as possible. Also, from George and Fred, a mournful wish to have brought more prank supplies with them, for if they were to go down, they'd go down fighting, nevermind that the contraband would be further evidence against them.

And so, the Room of Requirement did its ever best to accommodate the desires of its thirty odd patrons, as well as the Inquisitors waiting impatiently in the hall. So it was that the doors of the Room opened. So it was that Dolores Umbridge came to enter an empty Room. So it was that she, Mr. Filch, several Slytherins and an innocent cat came to be covered by copious amounts of hive-inducing cranberry pie filling. So it was that the thirty odd students who comprised the DA vanished from Hogwarts, never to be seen again.