Twilight was in her library playing with some potions and other science things. She had found a hidden book in her library and decided to read it thoroughly.  And then she stumbled upon this one potion that had a lot of the description smudged except for a part where it says it cancelled out magic and the ingredient list was under it.

So she thought it was smart to make this potion just as a test of her capabilities as a pony of science. It took her hours and hours to get it right. And then she finally did.

"I've done it." she says holding up the vile,

And then Pinkie bursts through the door, causing Twilight to drop the potion.

"Damn it Pinkie, I spent all day on that!!" Twi yells at her pink friend.

"I'm sorry." Pinkie replies.

"now I have to start again." Twi says ruffling her mane with her hooves.

She tries to pick up the broken glass with her magic, but her magic wasn't activating.

"What the hell?" she says quietly while Pinkie hops around, "I guess it did say it was a magic canceller."

She sighs and sits on her couch and does some thinking. Little did she know, the spilled liquid began to bubble and turn into a thick tar like substance. It started to move around on it's own, and shot toward Pinkie, whom felt a small prick on her behind. She jumped slightly, rubbing the spot. But it only felt like an itch and she ignored it going back to her usual shenanigans.