If you are billing yourself as a writer of historical fiction, you should have at least a basic grasp of history. And then you need to do lots and lots of research on all the details of the time period you are setting your story in.
Some interesting historical "facts" we have seen in both submissions and, alas, published books.
1. Ah, those medieval feasts, with lords and ladies being served a turkey. Wonder how they got these North American fowl in fourteenth century England?
2. You did know that the zipper was common in ancient Egypt, right? Made it much easier to get into and out of those nifty robes.
3. The lord and lady of the medieval manor invited the countryside to their wedding - where they merrily danced the waltz. (The waltz originated in the late 18th century.)
4. The old Earl and his Countess hated their oldest son's bride, so when the Earl died, his wife declared that the title and entailed lands would go to their younger son! Perhaps the author should have done a bit more research on English inheritance laws. I threw the book against the wall.
5. Heroine accidentally breaks a precious vase. So she immediately goes on eBay and buys a replacement. This seems perfectly reasonable - except that this was a time travel, the heroine was now in the early 19th century. Electricity ? Computer ? Internet ? eBay?
6. Historical American Western - the heroine riding in a stagecoach rolls down the glass windows.
7. Setting is colonial New England, early 18th century; hero is an Indian, heroine an English lass. The heroine is injured, and the hero fixes her with a BandAid® !
8. In a Regency-set historical, the heroine describes her heart racing like an engine revving.
9. The mid-18th-century character was caught stealing food and transported to Australia. Um, no on both counts. It was to the Colony of New South Wales with them (starting in 1770), or Van Dieman's Land (actually Tasmania). The name Australia came into use around 1800. And transportation of convicts started in 1788 and went on for about 80 years.
10. A Regency miss was going to elope with her unsuitable suitor - and assured him she'd be awake at midnight to sneak out to meet him, because she would set her alarm clock!
11. Those American Revolutionary War soldiers only thought they were dying of cholera. Actually, cholera did not spread much beyond India before 1829.
12. So nice that the 1920s-era flapper drank Tang instead of something harder. Too bad Tang wasn't around until 1959.
13. Please, please don't have your pre-20th-century character using "cool" as a slang term to mean something fashionable or popular or desirable. A Regency miss wears a shawl when going out for a walk on a cool day and is cool to the undesirable suitor she meets on the way, but she does not describe her new bonnet as "cool".