The year is 1814, and on page 22, the heroine says to the hero, pointing to her dog, "He would not win any prizes at a dog show, would he?" And I think, Hmm, did they have dog shows in 1814? Maybe a Best Dog Contest at the village fete?
According to Wikipedia, "The first modern conformation dog show was held in Newcastle-upon-Tyne in 1859."
On page 26, the hero, a military man, thinks of all the bivouacs he's endured. And I think, Was that a word then?
According to the The Compact Oxford English Dictionary, "bivouac" was in common usage by the French Wars in the 18th century, and Wellington used the word in a document dated 1811.
On page 59, the hero refers to the heroine having a bleeding heart.
This one is tougher. The adjectival meaning of "bleeding" as excessively anguished, e.g., with compassion, dates back to Spencer in The Fairy Queen (1596), but the OED has no quote later than 1713, so it could have been an obsolete usage by 1814. The use of "bleeding heart" as a noun-phrase (e.g., "she's such a bleeding heart") or as an adjectival phrase (e.g., "a bleeding heart liberal") is from 1958 or 1960 at the earliest. (All from the OED.)
Also on page 59, he refers to her having filled her home with lame ducks.
According to Eric Partridge, A Dictionary of Slang And Unconventional English, a lame duck was a defaulter on the Stock Exchange, ca. 1760-1870; the use of the term to signify "anyone handicapped or disadvantaged" is from the late 20th century.
This is so not a big deal, but just because it's the sort of insanity I think about, here's a reminder: Under the Marriage Act of 1753 (aka Lord Hardwicke's Act), common law marriage was abolished and the age of consent was raised. Because parental approval was required, the banns were read in church on three successive Sundays so that anyone with any objections had adequate notice of the nuptials. In the alternative, the couple could obtain a license. They would only need a special license from an archbishop if they were planning to marry outside a church or other place sanctioned for the performance of marriages. This is pretty much still true in the UK, and when we got married at Fountains Hall (a place specially authorized as a venue for marriages), we had to get a license from the registrar in Harrogate.
Page 64: The hero rode up the driveway . . .
The word "driveway" is chiefly North American, and its use to mean the surface leading up to a house dates from the mid-19th century. (OED)
Okay, that's probably enough. Again, none of the above is a slam on this book or its author.
You may be thinking, "Oh, so you're one of those people," meaning black-hearted curmudgeons who find fault with everything. I may be, but that's not quite the point I wanted to make. I'm quite enjoying the book I'm reading. I don't mind a book with historical errors -- I doubt I've read a historical romance without any! -- but I would hate to make those errors myself. When I read something I worry is wrong, I cringe with empathetic anxiety. I realize all too well that "there but for the grace of the writing gods go I."
And here's the thing: I would either make tons of errors or I'd make none . . . on the 20 pages I actually managed to write! After about fact-checking 20 pages, I'd be ready for the loonybin. I don't like to research. It's taken me a couple hours just to look up the few words and phrases I've cited here -- and I'm lucky because I married a man with an entire reference library. And even if I managed to write one, I don't think I could relax, waiting for the smug email letting me know of my errors. (Hey, I may be a black-hearted curmudgeon, but I would NEVER write to an author to point out a trivial error. That's just mean.)
I admire authors who take on this challenge. And I'll never join their ranks.