Sectumsempra

All right, another one of J.K. Rowling's genius moments. If you haven't read Harry Potter and the Half-Blood Prince, you might not want to read this.
So Harry has this potion book that the Half-Blood Prince wrote notes in, right? Some of those notes were spells of the Prince's invention. Harry discovers the spell "Sectumsempra" labeled "for enemies". He wants to try it out on Ron, but Hermione is sitting right there, and since she doesn't like Harry's little book he has there, he doesn't do it.
Guys, do you realize how crazy this is? I realized this only the third time reading it. Harry wanted to use Sectumsempra on Ron. What if he had done it? He could've killed his best friend. Harry had no idea what that spell was. He only learns what it is after he uses it on Draco and accidentally slashes his chest open. Luckily Professor Snape saved him, but just think if Harry had used that spell on Ron right then. And Harry didn't even think back to that moment when he did the spell on Draco, or after he did it. J.K. Rowling just leaves the reader to figure it out for themselves, if they can. It might take a rereading, or a reading of this post, to figure it out. I mean, it took me three times and by that time I knew the Harry Potter facts like the back of my hand. But I just think that's crazy.