Harry did not stay to watch Slughorn splutter. He and Luna stayed after Professor McGonagall, who had taken up a position in the middle of the corridor and raised her wand.

"Piertotum¨Oh, for heaven's sake, Filch, not now"

The aged caretaker had just come hobbling into view, shouting "Students out of bed! Students in the corridors!"

"They're supposed to be you blithering idiot!" shouted McGonagall. "Now go and do something constructive! Find Peeves!"

"P-Peeves?" stammered Filch as though he had never heard the name before.

"Yes, Peeves, you fool, Peeves! Haven't you been complaining about him for a quarter of a century? Go and fetch him, at once."

Filch evidently thought Professor McGonagall had taken leave of her senses, but hobbled away, hunch-shouldered, muttering under his breath.