Addendum

Though if your setting doesn't resemble the early 18th century, I encourage you to reconsider.

When to Ignore Anachronism

You may think of "ha-ha" as an early 18th-century term, but go ahead and use it no matter what time period your setting resembles.

Watch Out for This Anachronism!

It's easy when you're writing your medieval-adjacent fantasy story to slip up and describe a bag full of treasure as being made of nylon. Oops! That's all right though. Find and replace "nylon" with "plastic," and everything's fixed!

Rank Hypocrisy

People get all worked up about turkey and potatoes in vaguely medieval fantasy, but they never say anything about coconuts, and when you break open the coconut there's a spirit inside that gives you magic armor that's obviously just power armor and the spirt becomes your sassy copilot. Can you believe these people?

Characterization

Make sure to give your characters hobbies and interests such as model trains. This is for fantasy. In science fiction, replace that junk with long descriptions containing phrases such as "tidal-locked." Nobody has time for space chess when there's speculation about genetic optimization of luck to drop on the page.

On Archery Terminology

Some people make the mistake of having archers in a fantasy setting "fire" arrows. They should of course "blast" them to "totally rock" the enemy.

Star Ocean: The Divine Force

More like Translation: Is Bad, am I right here fellas? Mayhap inserting a couple old-timey words and phrases here and there doth ill serve us all, expecially when it's somebody like Theo who speaks with disarming casualness. You know, Japan does have a past. People there can talk like samurai or Heian court officials if they want. So if they don't, take that as a hint.

Who's the Narrator?

Ask yourself that when you write your story. Even if it's an omniscient viewpoint who isn't a character, is that viewpoint speaking to contemporaries of the story, generations to come, modern audiences, or what? Comparing something to a helicopter, for example, is definitely out of place for Drizzt to say, but not necessarily for the narrator. What kind of references would the narrator use, or what actions would he applaud or condemn?