The text is cute and imaginative, but it isn't a complete story. There's no resolution and the ending is rather flat, with Twilight's friends and Twilight having marginal insight into her having a problem and nothing else happens. We still don't know what the book is or where it came from, and the effect of the spell was too subtle for comedy to shine through.

You should put the word "off" in the last transposition somewhere.

Ha? Sorry, but this is pretty forgettable. Sleep deprivation works as an excuse for Twilight casting an unlabeled spell, but the end result just isn’t funny.

“Throw away a book?” Twilight looked at him as if he spouted heresy.
Heh, that made me laugh. I also liked some of the language you used in this. There isn't really a strong enough connection between Twi's motivation and the outcome/punchline for the latter to really work, but I appreciate that the word limit may have had some role to play in that. It does need a more complete resolution, however.

Thanks for sharing your work.

This story, unfortunately, seriously aggravated my inner pedant. In the abstract I like the concept here — a miscast spell affecting speech — but the spell has a flagrant lack of consistency that ruins the jokes for me:

I’m going to hay the hit

This is pretty much your establishing line, which means that it's a spell which creates … uh … whatever Spoonerisms are called when you swap whole words instead of initial letters. (Is there a term for that? There's got to be a term for that.)

a piece of doozy

But if you follow that rule, then Twilight's first verbal gaffe came from her trying to say "a doozy of piece", or "a doozy of a piece". Huh? TBH, I have absolutely no idea what she was *trying* to say here! "Doozy" stands alone there — "that was a doozy" — but no other gaffe adds or subtracts words, just shuffles them around, and I don't know where "piece" comes in.

“So whose idea was it to have breakfast at hayburger?”

Another word-spoonerism: hayburger[s] at breakfast.

“The re-shelving went as usual. Except for this one book that Spike found that I’d never seen before,” Twilight said. “None of you dropped any books off, did you?”

And she goes an entire paragraph without any slips.

So the bullet bit me

This threw me hard. This isn't a word inversion, it's a subject/object inversion. Turning "I" into "me" means that the speech disruption is context-sensitive and grammatically correct. D:

I was really midnighting the burnt oil

And here, a noun is verbed and a verb is nouned. "Hay the hit" works because it's just an ordering switch; this is getting syntactically tangled. And you didn't even need to overcomplicate it! "I really midnight the burned oil" works equally well and is a simple transposition!

I think I chewed more bit than I could last night.

The colloquialism is "bit off more than I could chew", so you're losing words here too, and this is less a word switch than running the sentence through a blender.

Look, maybe this is just editor OCD coming to the fore, but this doesn't read like magic to me. Magic has rules. You change one thing, and that one thing changes. Maybe the premise doesn't make sense, but it's self-consistent. Poison Joke makes Applejack tiny, and it makes Twilight's horn floppy — but we never see Twilight shrink, and we never see AJ's hooves wilt.

Maybe, maybe, I could accept this as-is if it were explicitly, in-story lampshaded as a prank by Discord — because he has a way of making consistency go out the window. But it would be so much more satisfying if you worked out the rule of your spell ahead of time — "the magic switches two adjacent or nearly-adjacent words in each sentence, with no other textual changes" — and then crafted your dialogue to take full advantage of your basis for comedy.

tl;dr: There are long stretches without changes that are crying out for further wordplay, and the ones that you've got are place over the all. Please, please, up this clean and premise with the run — because I think the premise has spades in potential.

Tier: Needs Work

This story is unbalanced and unfocussed. What's funny in it is Twilight’s induced dyslexia, isn’t it? Then why devote more than a half of the story telling us how she puts a hoof on that strange book? You should start – as >>horizon once said, quoting Kurt Vonegut – as close as the end as possible, showing us Twilight walking up in the morning and fumbling her sentences, then maybe a flashback to that strange spell casting.

Well, the idea is fun though. But I think the idioms you took do not lend themselves pretty well to the gag. Chose some idioms where the inversion really botches up the sentence in a funny way. I.e. It’s raining cats and dogs ↔︎ It’s raining dogs and cats. Oops :P