Settled in the story
How did the Death Eaters get into Hogwarts in the sixth book?
Peeves smashes one cabinet as a favour in book two. A bully vanishes into it in book five and comes back with a story. By book six, Draco Malfoy has turned the pair into the hole in Hogwarts' defences.
This thread gives away a resolved story point
The clue
A matching pair of cabinets, one in a junk shop and one in a castle — smashed in a prank, tested by a bully, and repaired into a doorway the whole war walked through.
The trail
- Book 2 · The Deathday Party
To rescue Harry from Filch's office, Nearly Headless Nick talks Peeves into dropping the school's vanishing cabinet from a great height directly overhead. It lands in splinters: a joke, a distraction, a piece of furniture destroyed in a corridor prank.
- Book 5 · Montague's disappearance
Fred and George shove Graham Montague, Inquisitorial Squad bully, headfirst into that same damaged cabinet. He is gone for days, then reappears half-drowned in a toilet, raving about being trapped between places — and about voices he could hear from a shop called Borgin and Burkes. The cabinet, even broken, goes somewhere.
- Book 6 · Draco's Detour
In Borgin and Burkes, Draco examines the cabinet's twin with unusual care, warns the shopkeeper to keep it safe, and mentions something of his own that needs mending. A shopping trip, read again later, turns out to have been a site survey.
- Book 6 · The Lightning-Struck Tower
The repair is finished. On the night Dumbledore dies, Death Eaters step into Hogwarts out of the Room of Requirement as if through a side door — because that is what Draco spent a miserable year building: a passage between a junk shop and the school that no ward was watching.
The payoff
The two cabinets formed a passage; Montague's accident proved the route existed even in its broken state; Draco's entire sixth year was the repair. The castle's famous protections were never breached. They were bypassed by furniture.