Recently I read HPMOR and this paragraph was one of many delights there:
The cold in the room seemed to deepen. “A sixth-year Gryffindor cast a curse at one of my more promising students, a sixth-year Slytherin.”
Harry swallowed. “What...sort of curse?”
And the fury on Professor Quirrell’s face was no longer contained. “Why bother to ask an unimportant question like that, Mr Potter? Our friend the sixth-year Gryffindor did not think it was important!”
“Are you serious?” Harry said before he could stop himself.
“No, I’m in a terrible mood today for no particular reason. Yes I’m serious, you fool! He didn’t know. He actually didn’t know. I didn’t believe it until the Aurors confirmed it under Veritaserum. He is in his sixth year at Hogwarts and he cast a high-level Dark curse without knowing what it did.”
“You don’t mean,” Harry said, “that he was mistaken about what it did, that he somehow read the wrong spell description—”
“All he knew was that it was meant to be directed at an enemy. He knew that was all he knew.”
And that had been enough to cast the spell. “I do not understand how anything with that small a brain could walk upright.”
“Indeed, Mr Potter,” said Professor Quirrell.
There was a pause. Professor Quirrell leaned forward and picked up the silver inkwell from his desk, turning it around in his hands, staring at it as though wondering how he could go about torturing an inkwell to death.
“Was the sixth-year Slytherin seriously hurt?” said Harry.
“Yes.”
“Was the sixth-year Gryffindor raised by Muggles?”
“Yes.”
“Is Dumbledore refusing to expel him because the poor boy didn’t know?”
Professor Quirrell’s hands whitened on the inkwell. “Do you have a point, Mr Potter, or are you just stating the obvious?”
“Professor Quirrell,” said Harry gravely, “all the Muggle-raised students in Hogwarts need a safety lecture in which they are told the things so ridiculously obvious that no wizardborn would ever think to mention them. Don’t cast curses if you don’t know what they do, if you discover something dangerous don’t tell the world about it, don’t brew high-level potions without supervision in a bathroom, the reason why there are underage magic laws, all the basics.”