These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: MLP: Friendship Is Magic S1 E10 "Swarm of the Century"
Accidental Aesop: It's meant to be about listening to your friends' advice, but becomes more about the importance of explaining yourself properly, given that Pinkie hardly even tries to explain why she's looking for musical instruments.
Or warn the others, upon seeing the first few parasprites, that they're bad news. Or explain that she IS trying to solve the problem, at any point. There were a lot of opportunities for her to have explained herself. Oh, Pinkie Pie, you could have saved everybody a lot of trouble if you weren't such a Genius Ditz.
Also, as mentioned below, one could take it as "don't take animals you don't know anything about out of their natural environment".
Fridge Horror: There was a reason Pinkie Pie's family had nothing to farm but rocks.
The parasprites in general. Sure they were herded away from the town, but that doesn't guarantee they won't stop multiplying. For all we know, they could be still out there, slowly increasing in number, untill... well, best not to think about it. Unless the Canterlot ponies have, ahem, 'methods' of keeping their population down... which is kinda Fridge Horror in inself considering how cute they look.
Or if they just have natural predators in the Everfree Forest.
It makes it become another Accidental Aesop about "invasive species". Don't put animals away from their natural environment, kids.
We're suppossed to assume the parasprites herded away by the Mane Six have returned to the Everfree Forest, but who's to say they did? It is not hard to hypothesize they kept on going until they hit some other town, and Celestia allegedly leaves because Fillydelphia caught an infestation of "bothersome creatures". Now, we can either stay positive and assume the actual reason is that she figured out what's going on and is sparing them the embarrassment, or...
Fortunately, according to the official map◊ of Equestria, parasprites that were driven out of Ponyville into the Everfree Forest and decided to keep going in a straight line would not find themselves in Fillydelphia. It would be Appaloosa finding themselves under invasion.
Inferred Holocaust: Read the first quote on the main page. It seems Zecora's people have experience with the little monsters.
Memetic Mutation: You're not supposed to feed parasprites, and you're not supposed to "feed" internet trolls. Thus the brony community rather quickly started calling internet trolls parasprites.
Nausea Fuel: During the parasprites' rampage in Rarity's shop, the sounds of several parasprites vomiting can be heard. It's as gross as it sounds.
Padding: The scene in Zecora's hut is considered by some to be mere filler, as Zecora merely tells Twilight that Ponyville is doomed (as the town is already being destroyed) and contributes very little else to the rest of the plot.