One of my pet peeves is similar, namely writers who simply can’t write for the canonical setting. This actually has a fairly wide scope, and OOC can be a side-effect.

Harry Potter fans in rural Alabama transferring the whole setting from 1990s’ England into modern-day redneck country and having everyone speak y’allbonics because that’s all they know. Bonus points for stuff like the Weasleys’ home having been destroyed by an F5 tornado, nothing close to which ever occurs in Great Britain.

Stranger Things fanfic with the same setting as the TV series, but suddenly, everyone has current-day smartphones and uses Instagram, Discord and TikTok because the author can’t imagine what life without all that used to be like. Bonus points for references to early 2020s’ pop culture.

Similarly, elements from the real-life human world (smartphones, certain websites/online services, Christmas) in settings where they shouldn’t exist because they’re sci-fi, fantasy or otherwise not taking place in any imaginable version of this world. This is even more blatant than writing every place in the world like it’s the USA with all its culture. Imagine the canonical characters in a Miraculous Ladybug or Harry Potter fanfic celebrating the 4th of July. Now imagine the characters in a Game of Thrones, Lord of the Rings or Star Wars fanfic doing so.

Something I’ve actually seen is My Little Pony: Friendship is Magic fanfic in which the ponies are written as ponies but like humans, and Equestria is much closer to the human world than in canon. Not only can ponies do everything with their front hooves that humans can do with their hands because the author is incapable of going with the limitations that come with hooves, but there are also things like guns that don’t exist canonically in Equestria, that wouldn’t fit into the setting anyway and that’d be next to impossible to operate without fingers and opposable thumbs (never mind that so are many musical instruments in the show). Bonus points for clopfics in which ponies have human anatomy between their hind legs and have sex in human positions. I mean, the author could have made it an Equestria Girls fanfic instead, but they hate Equestria Girls. Or they could have written the characters as humans or anthro, but they don’t like the former either, and they’ve never heard of the latter.

And something I think I’ve gladly never come across is when fanfic in mousepunk settings like Chip 'n Dale Rescue Rangers makes the characters human-sized in either a fully anthro setting or even the human world because the author can’t write the size difference, or because the author wants to ship their human self-insert with one of the rodent protagonists while not bothering with creating a fursona. I mean, I’ve written the Rangers human-sized myself, but with a very good explanation and in a setting rather unfamiliar to themselves. I’m actually pretty certain that there’s sloppily-written Zootopia fanfic that completely disregards species sizes.