From HBP:
There was a loud bang and the bin behind Harry exploded; Harry attempted a Leg-Locker Curse that backfired off the wall be-hind Malfoy's ear and smashed the cistern beneath Moaning Myr-tle, who screamed loudly; water poured everywhere and Harry slipped as Malfoy, his face contorted, cried, "Cruci —"
"SECTUMSEMPRA!" bellowed Harry from the floor, waving his wand wildly.
Blood spurted from Malfoy's face and chest as though he had been slashed with an invisible sword. He staggered backward and collapsed onto the waterlogged floor with a great splash, his wand falling from his limp right hand.
"No —" gasped Harry.
Slipping and staggering, Harry got to his feet and plunged toward Malfoy, whose face was now shining scarlet, his white hands scrabbling at his blood-soaked chest.
"No — I didn't —"
Harry did not know what he was saying; he fell to his knees beside Malfoy, who was shaking uncontrollably in a pool of his own blood. Moaning Myrtle let out a deafening scream: "MURDER! MURDER IN THE BATHROOM! MURDER!"
The door banged open behind Harry and he looked up, terrified: Snape had burst into the room, his face livid. Pushing Harry roughly aside, he knelt over Malfoy, drew his wand, and traced it over the deep wounds Harry's curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy's face and repeated his spell. Now the wounds seemed to be knitting.
Harry was still watching, horrified by what he had done, barely aware that he too was soaked in blood and water. Moaning Myrtle was still sobbing and wailing overhead. When Snape had performed his countercurse for the third time, he half-lifted Malfoy into a standing position.
"You need the hospital wing. There may be a certain amount of scarring, but if you take dittany immediately we might avoid even that.. . . Come..."
He supported Malfoy across the bathroom, turning at the door to say in a voice of cold fury, "And you, Potter . . . You wait here for me."
It did not occur to Harry for a second to disobey. He stood up slowly, shaking, and looked down at the wet floor. There were bloodstains floating like crimson flowers across its surface. He could not even find it in himself to tell Moaning Myrtle to be quiet, as she continued to wail and sob with increasingly evident enjoyment.
Snape returned ten minutes later. He stepped into the bathroom and closed the door behind him.
"Go," he said to Myrtle, and she swooped back into her toilet at once, leaving a ringing silence behind her.
"I didn't mean it to happen," said Harry at once. His voice echoed in the cold, watery space. "I didn't know what that spell did."
"What would your first intuition be, Mr. Potter? Surely even with your rudimentary English skills you could determine what Sectumsempra does. Where did the Boy Who Lived learn such a curse?" Snape asked, his face betraying no emotion.
"I... read it somewhere. I didn't know what it did. I thought it would be an annoying prank spell like Levicorpus... I hoped it would distract Malfoy from using the Cruciatus on me. I didn't mean to seriously injure him! I thought it was just a hex!" Harry exclaimed, shaking.
Snape frowned. "Tell me where you read it."
"I... I don't remember. I think it was from a book in the restricted section. It didn't mention what it did," said Harry, praying that Snape would not employ Legilimency to find the true place.
"Liar. Just like your father. You live to kill your hated Slytherins! Bring me your Potions textbook."
"But – why? It isn't - !" Harry cried.
"Bring it to me before I take you to the Headmaster and have you expelled!"
Harry hurried away and without asking Ron, stole his book. He quickly passed by the Room of Requirement, entered it, and found a magical eraser which removed the "Roonil Wazilb" from the cover. He quickly returned to Myrtle's bathroom, shaking like a leaf.
"Really, Mr. Potter, you cannot expect me to believe your feeble attempts at hiding this given your apparent emotional state. Since you are the Chosen One to defeat the Dark Lord, I will offer you this one concession. Instead of expelling you, I can employ corporal punishment upon your person along with the usual detentions. It is your choice" Snape said, in his usual demeanour.
"Sir?" Harry stuttered, horrified.
"Which will you have? I haven't got all day, Potter. I do need to brew some dittany for Mr. Malfoy's wounds."
"I'll have – I don't really have a choice, do I, Snape?" Harry fought back.
"That's right, you don't. Come to my office, Potter." Snape dragged Harry by his hair to his office in the dungeons. Harry hadn't been there since the Occulmency lessons ended. "Now, Potter, you will remove your shoes and jeans and lean over the desk, feet half a meter apart. Any time you move out of position, we will simply restart your punishment. You will count each stroke out loud before thanking me for each one. Hand me your belt. Now!"
Harry looked about to resist, but he decided against it. Quietly he unbuckled his belt and handed it to his most hated professor. Then he kicked off his shoes and removed his jeans, leaving him in a shirt and boxers. He had to summon some of his Gryffindor courage to lean over the desk as prescribed. Snape walked behind the desk so that Harry could see him, transfigured the belt into a formidable paddle, and walked behind Harry. He trembled in anticipation. He'd never been spanked so precisely before. Uncle Vernon and Dudley always just lit into him with whatever was laying around, regardless of Harry's position. Harry was jolted out of his thoughts when pain exploded in his bottom. He cried out because he hadn't been expecting it.
"Ouch! Ahhhhrgh bloody hell this hurts!"
"Count and thank me, you idiot boy!" snapped Snape.
"One. Thank you, sir," responded Harry, and no sooner had he said that than he felt a second stoke. "Owwwww fuck! How many more? Two, thank you sir."
"You are getting twenty. Would you prefer expulsion and Azkaban?" Snape replied, harshly dealing three strokes before allowing Harry to count them.
"Agghhhh fuuuck this hurts! Three thank you, four thank you, five thank you. Isn't five e-" Harry cried, struggling to get out of position.
*WHACK WHACK WHACK WHACK WHACK WHACK WHACK*
Harry sobbed, trying hard not to jump away from the desk. "T-twelve, sir. Merci."
Snape could see horrid bruising through Harry's white boxers. He dropped the paddle and whispered "Finite." (Unbeknownst to Harry, Snape had fastened him to the desk.)
Harry stayed in position, unsure what to do. He was sure the greasy git was just getting his hopes up, and he didn't want to be disappointed. Surely Snape had not let him off punishment! He began blushing from embarassment. What would Ron say, seeing Harry in this position? Oh, Hermione is going to say "I told you so" because of her warnings not to use the curse... Why didn't Hermione tell me what the curse did, I wonder, thought Harry. Why didn't she save me from...
"Well, Mr. Potter, you really do place a great deal of importance on yourself. Believe it or not, I have potions essays to grade, and I need my desk free of impertinent teenager for that task. Are you trying to aggravate me further?" snarled Snape.
Harry hastily jumped up, yelping as his buttocks reminded him of the painful paddling just a few minutes prior. He blushed as Snape openly laughed at his discomfort. "I'm sorry sir. It's just you said twenty and I thought..."
"By all means, if you want eight more, lean back over the desk and I'll do the honors. I thought you might be at least a little grateful for the clemency. By the way, the castle paintings will have spread the rumour now that Harry Potter received twenty swats on an unclothed bottom with a formidable holed paddle. So you needn't worry about telling your friends. Now leave, you abominable child!"
Harry had attempted to protest during this monologue but had found his mouth clamped shut. Anxiously, he gingerly walked from the room. He found Hermione and Ron waiting for him.
"Harry! What on earth happened? Is it true that Snape used corporal punishment on you? Are you okay? What did you do? Why is Malfoy in the infirmary? Corporal punishment is really serious – are you going to be expelled? Do you need to see Madame Pompfrey?" cried Hermione.
"I don't really need to talk about it, 'mione..." said Harry, visibly wincing at Hermione's sharp hug. Ron looked worried but hadn't gotten a change to put a word in edgewise during Hermione's monologue. "Let's go get lunch, okay? Unless you've already had yours."
"We haven't, Harry... Are you sure you're okay? You look like you've been, uh..." Ron supplied.
"I'm fine. Forget about it. Oh and... um I borrowed your copy of Advanced Potions. And lost it I'll try to get it back... I'm sorry Ron."
"How'd you lose it? What happened when you chased Malfoy? You've been gone for HOURS!" exclaimed Hermione.
"I'll tell you since no one is listening. The whole school probably knows though so you can hear it from them. I almost killed Malfoy. Stop grinning, Ron, this is serious! And it was an accident. I didn't know what Sectumsempra did, I thought it was like Levicorpus. Malfoy tried Crucio on me though... so I didn't have much choice. I found him crying like a baby to Moaning Myrtle. Snape found us and healed Malfoy with a song-like countercurse. He knew where I found the curse somehow and he told me I had the choice of corporal punishment from him or expulsion. That isn't really a choice so here I am."
"I told you that you shouldn't use that spell! Why didn't you listen to me? What if it was like Avada Kedavra? You thought it was a good idea to use a permanent cutting spell? I know we all hate Malfoy but you don't want to kill him!" Hermione cried.
"Please Hermione, Harry just got beaten for this, he doesn't need your censure too. Have you ever been paddled? The most I've gotten is eight with the brush from my mom, and that isn't nearly as bad as a holed paddle, nevermind that he got more than twice that! And I couldn't sit for a week!" retorted Ron.
"Um... I got twelve. Snape let me off the last eight for some reason. He said he'd increase the paddling if I left the desk, but he stuck me to the desk so I really couldn't make him go through on that threat. He seemed really mad at me though."
"Talking about me? Perhaps I should paddle all of you gossips soundly!" said Professor Severus Snape.