((Prompt: Rarity stops making a dress.))
The needle froze in its place, the tip just barely pricking the white fabric. Behind it trailed a line of white thread, held taut by a delicate and experienced field of magic. The final piece of the billowing white dress hung motionless. Waiting for the needle to strike forward in the last few stitches needed to make it complete.
Rarity stood by, similarly frozen. Her soft azure eyes fixed on that needle, the glimmer from her horn caught fast in their depths. The edge of a song seemed ready to burst from her lips as she focused on the last stitches of the wedding dress.
The stillness held the whole room in its thrall. Indeed, if Twilight cared to look out the window, she knew she would find the whole world stuck in a single moment of time. She did not care to look out the window. She looked only at Rarity.
Twilight stared for what seemed to be a very long time, but she knew for a fact it was less than an instant. She stared at the pearl and seashell earrings stuck in mid-dangle from her ears. She stared at the slightly disheveled mane that would be among the first things to go once the dress was finished. She stared at the carefully trimmed white hooves operating the controls of her sewing machine with expert precision, and she stared at the white-gold horseshoes set aside next to the door. She stared at the gem-studded ring placed carefully on a pillow next to Rarity's desk.
Twilight took a small breath, and her horn glowed a little brighter. No longer was the needle frozen. Instead, it moved backwards. The needle followed its own thread as it backed out of stitch after stitch. The white trim of the dress slid away and folded itself off to the side.
The needle continued moving backwards, unmaking the dress step by step. Seams were unsewn, pieces of cloth moved back to join with the soft linen as though they had never been cut from it. Gems and lace returned to their containers, and soon the dress was just a few rolled-up bolts of fabric and spools of thread.
Twilight's horn glowed brighter yet, and Rarity stepped backwards out of her Boutique. Celestia's sun rose in the west as Ponyville's night-owls rose from their beds, and set in the east, seeing all the morning-ponies off to dreamland. Monday followed Tuesday followed Wednesday followed Thursday.
Twilight watched Rarity in a fancy restaurant as she threw her hooves open, her embrace of... him... evaporating into nothingness. Her gasp of surprise reversed into a short bark of laughter as he took the sparkling ring away from her and tucked it into a box. He got up off his knees.
June came after July came after August came after September. Twilight looked on as Rarity looked at him one last, first time and then turned away, his name completely gone from her memory. He backed slowly away and left her standing alone on the dance floor.
Rarity sat at the bar, filling drink after drink as the bartender nodded with a compassionate but neutral expression, pouring the drinks back up into the bottle. She fled the bar, galloping backwards fast enough to remove the streaks her tears had left running in her mascara.
Twilight watched as Rarity threw herself backwards onto the bed, tears running up her cheeks and into her eyes. She watched as Rarity's eyelids became steadily less puffy and damp, until they were nearly dry once more.
She watched Rarity stomp back down the stairs. She watched as Rarity whirled around to face her. She saw the shock drain away from her face as those hurtful words were drawn from her ears like poison from a wound.
Twilight could swear she could see the air itself ripple as she unsaid what she had said to Rarity. There she stopped, and closed her eyes. Everything was still once again. She could feel the unmoving air catch the feathers in her flared wings. She could feel the hot sting of anger on her cheeks. She could feel her ears pressed flat against her skull.
From here, she would hold her tongue. She would quell those spiteful, nasty words, those words she had never meant to say. She would let Rarity win the argument. It wasn't worth it, for once, to be right. She would let her head cool, and let Rarity do the same, and she would never break her heart like she had done. Twilight would never drive her off like she had done. Rarity would never need to meet him. He would never need to heal her pain. He would never move into the place in Rarity's heart that had once belonged to Twilight Sparkle.
Twilight opened her eyes, and shook her head. Time spells didn't work like that. What had happened, happened. There was no changing the past. She knew that. Even if she could change it, she had no right to. He made Rarity happy. It was obvious—so very painfully obvious that he did. Even if Twilight were able, she could never take that away.
...Could she?
The needle pressed forward, finishing the final seam on Rarity's wedding dress. “There, done,” said Rarity, holding it up to the light. “My goodness. I don't mean to sound vain, but I daresay I shall have the most fantastic dress ever seen at a wedding, wouldn't you say?”
Twilight gave Rarity a smile. “It's beautiful, Rarity.”