Chapter 2: The Warning
"And finally, I must tell you that this year, the third floor corridor on the right hand side is out of bounds to everyone who does not wish to die a very painful death." Harry frowned. "Is he completely insane?" he muttered to Percy. "What do you mean, Harry?" Harry sighed. "There is something that can cause a very painful death in a school, and he just told everyone where it was, and not to go there. How long do you reckon it'll be before someone gets dared to go in there, or goes and looks out of curiosity?" Percy frowned. "Knowing my brothers, about a week, if that."
As it happened, nobody went into the forbidden corridor, because they didn't have the chance. Harry had not been the only one to pick up on Professor Dumbledore's warning, and quite a few students sent letters straight to their parents. Three days into the term, and a practical horde of adults marched up to the castle. Susan Bones spotted her aunt Amelia among them as they walked past the greenhouses, explaining to the other first years that she was the head of the Department for Magical Law Enforcement at the Ministry.
That evening at dinner, Professor Dumbledore announced that the third floor corridor was no longer out of bounds.