#but i believe that the suggested way Levicorpus got out is 100% canon compliant

[danadiadea]

The fact that within that theory Snape's language about Levicorpus is similar to Ron's and Harry's (even though they've seen Muggles tormented with it and Harry saw Snape tormented with it) is a neat observation.

Basically I think this is a solid theory, but then Lily's objections towards it being dark magic are even weirder, because this is... not true? Levicorpus isn't more of a dark magic than Wingardium Leviosa or Scourgify. It can be used to assault and hurt people, but... no-one ever thinks it's dark on it's own? Not Harry, not Ron, not Hermione, even if she rightfully points out it can be used to do harm, and evidently not James, who "hated dark magic" (but didn't hate illegal, so this is to be taken with a grain of salt), not Sirius or Lupin.

Lily's main (and kinda only) complain to what Mulcibier tried to do is that it was "dark magic". It's unfunny, according to her, specifically because it's dark magic, this is where she puts the empathis – and Severus doesn't disagree, preferring to change the topic. Now I already think that separating spells to abstract categories like this is dumb, and Severus clearly thinks so too, but why would Lily concentrate on a levitation jinx being "dark" when it's not, instead of bringing up that Mary had been hurt or forcefully undressed (I'm pretty sure Mulcibier tried to do something to her, not did, but maybe Lily meant he didn't take it as far as he wanted to) if she was? why wouldn't Severus disagree with her unjust classification? even if there are no acknowledged, clear borders between something recognised as "dark" or "light", not one other character who supposedly hates dark arts classifies Levicorpus as "dark".

Mulciber, Mary Macdonald and why Lily almost smiled in Snape’s Worst Memory

Note:  I appreciate that this is very long, but hopefully the theory is worthwhile:

Lily’s twitch in Snape’s Worst Memory has always bothered me.  

Lily, whose furious expression had twitched for an instant as though she was going to smile, said, “Let him down!”

Lily’s half reaction didn’t sit well with me when I merely thought she was the best bystander of a bad lot.

It deeply troubled me when The Prince’s Tale revealed that she was supposedly Snape’s best friend at the time.

Ever since The Prince’s Tale gave us some more context, I have not been satisfied by any explanation of Snape’s apparent loyalty to Mulciber.  His best friend tells him that his housemate is a creep, and Snape completely dismisses her.  

I was always confused by both of these reactions between two supposed best friends. 

We’re repeatedly told that Lily is a kind and caring person - so why would she be amused at witnessing her best friend’s humiliation at the hands of his hated enemies?

We also know that Snape cherished his relationship with Lily above all others - so why would he choose this moment to seemingly side with Avery and Mulciber over something so trivial?  As a consequence, it’s occasionally suggested that Avery and Mulciber had a Marauder-esque friendship with Snape, but canon doesn’t support this position.

However, there is one explanation that makes sense:

The Dark Magic that Lily Evans accused Mulciber of using was Snape’s created jinx of Levicorpus. 

The Half Blood Prince’s Potions Textbook 

Snape’s Worst Memory takes place during his OWLs.  During that scene, we see the use of three spells that later show up in the Half Blood Prince’s NEWT level Potions textbook as scribbled amendments. Snape was the creator of these spells:

the imaginative little jinxes and hexes scribbled in the margins, which Harry was sure, judging by the crossings-out and revisions, that the Prince had invented himself.

…so Snape must’ve been using the NEWT textbook in his OWL year.  This fits with Snape’s studious nature (as evidenced by him studying his exam paper), and given his apparently natural flair for Potions, it is logical that he would be challenging himself with wider reading.

It also means that his spells probably didn’t get into the public domain by accident; nobody would be borrowing his textbook to work from, as it was for a higher level.

So how did Levicorpus get out?

Harry and the Half Blood Prince’s Potions Textbook

When Harry discovers Levicorpus, he realises that it was a much struggled over spell:

Harry turned the book sideways so as to examine more closely the scribbled instructions for a spell that seemed to have caused the Prince some trouble. There were many crossings-out and alterations…

Look at what happens when Harry first unleashes it:

Pointing his wand at nothing in particular, he gave it an upward flick and said Levicorpus! inside his head.

“Aaaaaaaargh!”

There was a flash of light and the room was full of voices: Everyone had woken up as Ron had let out a yell. Harry sent Advanced Potion-Making flying in panic; Ron was dangling upside down in midair as though an invisible hook had hoisted him up by the ankle.

This is really important.  Harry points his wand at ‘nothing’ - and yet the spell finds Ron and hoists him into the air.

With this in mind, it seems fair to say that Snape could not have created this in isolation - even if he intended to; on casting the spell, it seems to pick up on the nearest human.

Who would have been nearest?  Presumably his housemates Mulciber and Avery - so that’s possibly how the spell got out.  

Let’s look at Harry and his housemates’ reaction to the spell:

“Sorry!” yelled Harry, as Dean and Seamus roared with laughter…

There was another flash of light, and Ron fell in a heap onto his mattress.

“Sorry,” repeated Harry weakly, while Dean and Seamus continued to roar with laughter.

Ron’s shock had subsided and he had decided that Harry’s new spell was highly amusing; so amusing, in fact, that he lost no time in regaling Hermione with the story as they sat down for breakfast.

“… and then there was another flash of light and I landed on the bed again!” Ron grinned, helping himself to sausages.

“And also,” she added, as Harry and Ron rolled their eyes, “because I’m starting to think this Prince character was a bit dodgy.”

Both Harry and Ron shouted her down at once.

“It was a laugh!” said Ron, upending a ketchup bottle over his sausages. 

“Just a laugh, Hermione, that’s all!”

“Dangling people upside down by the ankle?” said Hermione.  “Who puts their time and energy into making up spells like that?”

“Fred and George,” said Ron, shrugging, “it’s their kind of thing.”

And there we have it; it’s the sort of spell that’s deemed to be really funny amongst friends.  The boys see absolutely no problem with it - between mates, it’s a bit of a laugh.  Ron compares it to the practical jokes of his twin brothers:  decidedly NOT Dark Magic, but a jokey hex.  

It all comes down to intent:

“Maybe your dad did use it, Harry,” said Hermione, “but he’s not the only one. We’ve seen a whole bunch of people use it, in case you’ve forgotten. Dangling people in the air. Making them float along, asleep, helpless.”

Harry stared at her. With a sinking feeling, he too remembered the behaviour of the Death Eaters at the Quidditch World Cup. Ron came to his aid. “That was different,” he said robustly. “They were abusing it. Harry and his dad were just having a laugh.”

So, we can guess - given that some of Snape’s earlier spells were faintly harmless - that this was created in the same vein.  It was a jokey hex, intended to make people laugh - just like Fred and George creating a product for their shop.

But - like the Death Eaters at the World Cup - it’s how Mulciber uses the spell that causes the problem.  

When Lily confronts Snape, she says:

“I don’t like some of the people you’re hanging round with! I’m sorry, but I detest Avery and Mulciber! Mulciber! What do you see in him, Sev, he’s creepy! D’you know what he tried to do to Mary Macdonald the other day?”

There is a big difference between a boy throwing another boy in pyjamas up in the air in a friendly ‘having a laugh’ manner between mates in the confines of their dormitory…and a boy throwing an unsuspecting teenage girl - who he may not know very well - in robes up in the air in the middle of the school.

In the dormitory, the boy would be relatively harmlessly turned upside down; the girl may not have much on beneath her robes - and amongst teenagers of 15/16, the “joke” becomes sexual in nature…and becomes, as Lily suggested - creepy.   

If this theory is correct and Mulciber used Levicorpus, it’s possible this is how the spell escaped into the mainstream - and how James et al picked it up.  

Snape had taken great care to remark in his book that it should be used nonverbally, but if Mulciber used it recklessly and Mary heard the spell - well, is it any wonder that one of the Marauders managed to wheedle the incantation out of their fellow housemate?  Maybe they suggested they would gain retribution for her, but needed to know the spell so they wouldn’t fall victim to it.

After all, when Snape is attacked in Snape’s Worst Memory, it is curious that he’s wearing nothing but underpants beneath his robes.  It’s summer, so perhaps he’s hot.  He’s poor and it’s the end of term, so perhaps he’s grown out of his trousers.  

…but mostly, I think he’s entirely blindsided and not expecting other people to know the jinx.  If he had, I think he’d have been wearing more beneath his robes.

When we look at Snape’s reaction to Lily’s accusation, he says:

“That was nothing,” said Snape. “It was a laugh, that’s all – ”

His language is identical to Ron and Harry’s; it was just a laugh. 

Incidentally, the same is true of James, Sirius and Peter when they unleash it on Snape - just like Dean and Seamus, they roar with laughter:

Many people in the small crowd watching cheered. Sirius, James, and Wormtail roared with laughter.

But I digress.

The point is, Snape desperately didn’t want Lily to discover that he was the source of the spell that had been used in a creepy manner, because he hadn’t designed it to be used in such a fashion.  

He had genuinely designed it with harmless intent - but he soon saw that it could be used maliciously; and as Hermione pointed out, the Death Eaters continued to do so well into the 90s.

Lily calls the spell Dark Magic:

“It was Dark Magic, and if you think that’s funny – ”

“What about the stuff Potter and his mates get up to?” demanded Snape. His color rose again as he said it, unable, it seemed, to hold in his resentment.

On first read, it seems slightly weird that Snape leaps straight to talking about James Potter - but if we look at Levicorpus as being an ‘innocent’ spell and Lily accusing it of being Dark Magic, it’s easy to see where Snape makes the connection in his head.

After all, look at their next exchange:

“I’m just trying to show you they’re not as wonderful as everyone seems to think they are.”

The intensity of his gaze made her blush.

“They don’t use Dark Magic, though.”

And that’s Snape’s point; James et al don’t use Dark Magic, but their “jokey” spells are enough to bully him.  When does a spell stop being a joke and start being Dark Magic?

Furthermore, Sirius and James constantly bleat that Snape is a wannabe Death Eater and uses Dark Magic - so Snape wouldn’t want Lily to think that he was the mastermind behind a Dark Magic curse.  

So, Snape defends Mulciber because Snape invented Levicorpus, and he doesn’t want Lily to think badly of him.  

But why does Lily almost smile?

Because when she complained about Mulciber using the spell on Mary, Snape completely dismissed her and told her it was just a laugh.

I think that Lily saw Snape on the receiving end of it and a small part of her - even though he was her best friend - thought triumphantly that her point had been proven, “You see!  Now you don’t think it’s funny!”  

Alas, the irony was lost on Snape - and in his fury, he blurted out the slur that would end their friendship.