Not so much a fanfiction gripe as a my-stupid-brain gripe; I keep reading "quaking" as "quacking." (Waaait, leaves don't quack!) (why is the protagonist's body making duck noises.)
American writers throwing American animals into a British setting, or having characters in a medieval British setting be familiar with American species.
Sidenote: I wish "canon typical shipping" was an actively used tag the way "canon typical violence" is. Because a lot of canons have a low-key Obligate Het Ship, and when that's tagged in fic I never know if it's intended to be "this is tagged ebcause they're together in canon" or "this is a shippy fic focused on [ship]."
Potatoes are native to the Americas, as are tomatoes (those are the two I see most often transplanted to places they shouldn't belong). Putting them in a fantasy world like middle earth is less of a writing fail (edit: womp womp, I don't know lotr as well I ought, apparently it is supposed to be past Europe), but if you're writing about medieval England and your farmers are growing potatoes on their farms, that's not the way history would have actually worked.
Um Elves? Dragons? Angels? I understand breaks in reality but I'm confused. Unless it's funny. In which case wow that's an impressive research messup considering all the worldbuilding. In which case hi autism brain I guess.
It's more like it's a tiny little mistake that should have been easy to head off with a little research, but it's such a little detail that the authors who mess it up don't tend to realize that it needed research in the first place. When I was in Ireland, pretty much all the tomatoes I bought were grown in Holland, and potatoes are The Archetypical Irish Food, but that all is a pretty recent development history-wise. If you have the background to know that it's wrong, it's a little detail that can throw you out of the story in an 'aw, come on' way. Or like, my grandparents took me to a medieval dinner thing where it was in an old castle, period clothing, period music, period food. But when they brought out the soup, my grandpa asked what was wrong with this picture. I caught that tomatoes were a new world crop, but he also gave me a brief history of utensil use in Europe (tl;dr, spoons and forks were out there, but weren't in common use in Europe until roughly the 1600s). So if your medieval peasants are eating their soup with spoons, it's a harmless detail, but if you have the right background, it's gratingly wrong.
Though worth remembering it's only in the US that "Corn" exclusively refers to the maize family. Thus you can see references to corn in historical English things, but there it's a reference to grains in general.
I actually just went on a rant the other day about a similar utensil-research fail in Fate/Stay Night. There's a bit where Saber says she prefers chopsticks to a fork and spoon. Saber is Spoiler King Arthur. She has probably never even seen a fork in her life. (I declared my headcanon to be that she just said that to preserve her "foreign tourist" cover, and maybe if she tells the Japanese people she's eating with that she likes their weird new utensils more than this other weird utensil she was summoned knowing about, they'll like her better? :::PPP ) But if they mention ears of corn, that's a problem.
To be fair, I think the only time I've seen popcorn in medieval-ish fiction was "banged grains" in Discworld and Discworld doesn't count. :::PPP
Consider the following theory re: potatoes in Middle Earth: Gandalf gave them to the hobbits after one of his trips back and forth.
Or cases where the animal has reason to be there, but the characters don't have reason to know what it is. There's an actual case of this in real life; there's a town in England which was allegedly terrorised by a dragon, but the lord had a menagerie, and the stained glass window commemorating it depicts what is very obviously a crocodile. Point being, yeah, there may be an invasion of foreign reptiles going on, but when you have a character who's lived all his life in a setting which mashes together periods from Celtic to medieval with emphasis on the Viking invasions and a splash of 1940s military but all of those periods are obviously from British history, he's not going to be able to comment on the difference between a milk snake and a coral snake. And since it's Redwall and he's a rat, he should also probably be running away from a milk snake.
