The passage of time and slight changes to text formatting have not been kind to my story.

@LittlePipIsTheBestPip  
You’re welcome, definitely-best-Pip.

@Door Belle  
That was actually a pretty good story, thanks!

@Twilight_Shimmer  
What didn’t filly Anon do this time?
 
Wait, was I not supposed to answer tha
 

 
“Nonny.”
 
Twilight looms over you, her gaze a glare that could kill a horse. You swallow, glad you’re still a human on the inside.
 
“How many times, Nonny?” Twilight begins pacing the crystal castle’s library floor. She doesn’t watch you directly. She doesn’t need to. You both know you’ve learned not to run. “You do not forge ID to get drunk. You do not take advantage of Berry Punch being drunk to get drunk. You do not get drunk, period! You certainly don’t get drunk, throw up on several important pages of valuable magical tomes, and pass out on top of them! Do you have any idea what can happen? Besides destroying important books, you could have blown up Ponyville! You could have blown up Equestria!”
 
Internally, you chuckle. It’s almost like filly-barf is magic. You don’t smile. Twilight gets really annoyed when you smile during her lectures.
 
“What do you have to say for yourself?” Twilight stops pacing to glare at you again. You feel yourself shrinking into the floor. Not literally. Twilight actually shrank you the time she found you frying ants with a magnifying glass, and afterwards made you apologise to Fluttershy, who hadn’t even been there. It had actually hurt her to find out. Her tears did make you feel guilty enough to apologise for real. This doesn’t. Twilight’s just scary when she’s mad, less because of anything she has done than what you know she can do. And she’s a lot bigger than you are.
 
You swallow and don’t say anything. You know a lot better than to sass Twilight by now, and you know that if you open your mouth, you’re going to drop a vulgar joke or try to tell her off, which will just make things worse. And fake apologies are worth their weight in belly-button lint. Geez, you miss having a belly-button now. Maybe you’re getting used to it in some ways, but being a tiny girl horse is still weird.
 
“Don’t give me the silent treatment, Nonny.”
 
You stay quiet. You have nothing to say that won’t get you in bigger trouble.
 
“Fine. As soon as you’re washed up, I’ll talk to the girls.” Twilight snorts as she walks around behind you. You snicker. She’s funny when she makes actual horse noises.
 
“You’re having a sleepover tonight. You, the Crusaders, Fluttershy. It’ll be great.”
 
“Wait, no, I’m-what?” You’re cut off by how she lifts you by the back of your neck with her teeth. Your whole weight hangs there, but it’s not painful. Just… weird. Your whole body feels like it’s turned into a physics ragdoll from some game. You don’t control it–gravity does. What is this? Why is this? You barely register that she’s carried you off to the bathroom to clean up.
 
You tolerate the bath. It sucks when Twilight decides she has to do it for you, but then again, so does passing out where you did. And now you smell a lot better, even if Twilight does use the lavender-scented shampoo you hate. -It doesn’t even go with your colour Yeah no, you’re not going to let Rarity’s lessons go to your head. You’re a dude. You don’t, you won’t, know the first thing about colour-scent coordination. You look like a walking biohazard, anyway. What even goes with black and green? You’re definitely not going to ask Rarity. Maybe Sweetie Belle. She gloats less.
 
Wait. Sweetie Belle! Twilight’s taking you to an impromptu Crusaders sleepover! You flail in the bath, splashing water around. You’ve got to escape! Your masculinity can’t take being swarmed by those fillies again! Scootaloo’s pretty chill when you can get her alone, but in a group…
 
“You can’t do this! I’m gonna die there!” you say.
 
“Just let me know when you’re done fussing,” Twilight says, seemingly bored by the minimal effort of holding you relatively still in the tub. You struggle some more, but you’re just not big enough to out-muscle her, and she’s got magic. Yeah, you’re not going to win here.
 
“Fine,” you say. You stop fighting. You can always try to escape the sleepover when you don’t have someone way bigger than you who’s totally willing to use just enough force, or hack the world with her stupid spells, to stop you.
 
After another few minutes of washing and rinsing, Twilight pulls the stopper from the tub, but pauses, then sighs instead of picking you up and out of the swimming pool-like bathtub. It’s totally the size of an ordinary tub, probably. You’re just that small now. It’s awful.
 
“I’m sorry I yelled, Nonny,” Twilight says, hanging her head slightly. “Grown-ups shouldn’t yell at little fillies.”
 
“I’m not-” You stop when Twilight covers your mouth with a hoof. Ugh, that always feels so weird. The frog of her hoof is textured funny.
 
“It’s rude to interrupt, Nonny.”
 
Only when you do it, apparently.
 
“Now, I’m still taking you to the sleepover. I’m still a little mad and you need to learn your lesson and maybe make some friends on the way. But you do understand I’m doing my best, right? I want to see you grow up–again–happy and healthy here.”
 
You fold your arms. You still call them arms. Just because you walk on them doesn’t make them legs, clowns prove that.
 
“You can just send me back.”
 
“If I could, and I can’t, you’d get to your ‘Earth’ in the same body. I can’t undo that, Nonny, and you told me exactly why it would be a bad idea to send you back like this.” Twilight sighs and levitates you above the tub, running a brush through your hair and then flash-drying you. “Now, you’re not going to hit anypony at the sleepover this time, or there will be consequences. Understand?”
 
“Oh come on!” you say. Why is she still on about this? It was last year, and no harm was done. “Apple Bloom’s a ninja or something, I couldn’t even-”
 
“No. Hitting.” Twilight’s steely-serious face is out. You swallow. Yeah, you’re not running afoul of that.
 
You nod. She smiles pleasantly.
 
“Good. Now, let’s get going. I have a lot of walking and talking to do this evening because of you.” She shifts you into that weird scruff-hold again and carries you out of the bathroom. Then out of the whole wing of the castle. Then towards the massive front doors.
 
“Twilight. I can walk,” you say. She doesn’t answer. “Twilight! Can you at least teleport or something?”
 
She swings the right door open and steps out into the busy, sunset-lit streets, still carrying you.
 
“Oh come on!”
 

 
… the actual story is linked in the source comment, though.

Okay, what did Filly Anon do this time?