In terms of anachronisms, pockets are the bane of my existence. Pockets didn't begin to be sewn into clothing until the 18th century, and when you're writing fantasy that's "around" the 15th century, well... no pockets. I decided I didn't care, because it's fantasy and I doubt anyone would fact-check me, but I had a bit of a procrastination crisis over it.
I'll admit that I just let the pockets slide in pre-industrial era, secondary-world fantasy. There's no reason they
couldn't have gotten the idea before the 18th century and sewn pockets into coats, skirts and trousers, after all. Things don't have to happen exactly the same way in a fantasy world, and I make my own modifications to fashion and culture anyway.
For me, my love of certain aesthetics will drive these kinds of decisions. I'm too in love with the image of a certain characters slouching into the room with their hands in their pockets. Which is one reason I enjoy fantasy.
With purely historical fiction, or historical fantasy that is meant to take place in the real world, then one is stuck combing for those kinds of anachronisms.
I do take some pains to omit idiomatic references that a reasonably well informed reader will associate with a real-world person, place, thing, or event, like someone making a "Herculean" effort or something. But there's always a blurry line, since we also employ the translation principle when writing in a fantasy setting (or a historical time or place where they spoke a different language). Sometimes the desire to avoid anachronisms can go overboard, especially with words like "volcano," where there really isn't an alternative word.
I've just gotten through the first five or so chapters of a hilarious wonderful book called MEDIEVAL UNDERPANTS AND OTHER BLUNDERS by Susanne Alleyn, which gives all sorts of examples of anachronisms to avoid, including tomatoes in ancient Rome. It's a hoot!
Another problem is some things we think we know about the way people lived in earlier times turns out to be possibly untrue (like the
assumption that it was
the norm for girls to be married in their teens before the 20th century), or that
no one ever bathed in the middle ages.
There is some evidence that some women, at least, used undergarments
akin to a modern brassiere. Whether this was commonplace or not is up in the air, but in times and places when people made many of the things they used in daily life from scratch, some individuals might have gotten the idea to modify their own clothes in various ways.
Maybe some women did wear underpants (or clouts) of a sort, at least at certain times of month or if riding astride.
This kind of thing can be a real headache for writers of historical fiction who
did do their homework about what was plausible in a given time and place, yet people tell them they are wrong.
A bit like the reader who told me they couldn't possibly have microscopes in an early modern type setting. They could and they did, though they weren't nearly as effective or versatile as modern ones. There's a challenge with using a word (and the word microscope goes back to the early 17th century too) that most people associate with something very modern while showing the reader what the implement was actually like in that kind of setting.