Longer ago than I'd like to admit, when I was a teenager just beginning to read romance, I regularly encountered all the following scenarios and accepted them without question:
1) Lord Ravenwolfscar and Miss Pureheart can inherit a fortune...but only if they marry within six months of the death of Ravenwolfscar's uncle, Lord Manipulative-Moneybags.
2) The year is 1740, and our braw hero Hamish is instantly recognizable as a member of Clan MacSexylegs because he invariably wears his family's own distinctive tartan.
3) Lord Unlucky is dying without a legitimate son, so he weighs the worthiness of assorted nephews, cousins, and his own bastard son to determine where he should leave his titles and lands.
4) Lord Hesitant and Miss Doubtful must marry to protect her from an unwanted suitor, but they promise themselves not to consummate the marriage so they can have it annulled and go their separate ways once the danger is past.
Thing is? None of these scenarios could've happened, at least not in the Georgian/Regency milieu I currently write in.
1) The law frowned upon coerced marriages, even if the coercing was being done from beyond the grave. What Lord Manipulative-Moneybags could do was include a clause in his will saying Lord Ravenwolfscar must marry by his 30th birthday to claim the fortune without specifying a bride, which still gives the author plenty of room for plot conflict.
2) While the kilt itself has been around since the 16th century, formalized, clan-specific tartan setts date back no earlier than the Victorian era and were something of a marketing ploy. There were regional tartan patterns earlier on, so a group of, say, Gordons might all be wearing the same tartan because they all got it from the same weaver, but it wouldn't have broken a rule or been a breach of etiquette for a Fraser or Macdonald to wear the pattern as well.
3) Who inherits a title when a man dies without a legitimate son is a blog post in itself, but suffice it to say it's not the dying lord's choice, and he can't legitimize a bastard child. (Note that birth is what counts, not conception--if he marries his pregnant mistress as she's going into labor, the baby can inherit.)
4) The law didn't allow annulments for non-consummation. If the husband were impotent, incapable of consummation, it'd be a different story. But having spoken the vows, the couple couldn't just get a takesie-backsie by saying, "Oops, we didn't really mean it, and we haven't Done It, we swear!"
I expect to get some reader comments on #4 when my November book, An Infamous Marriage, comes out, because the hero and heroine marry in Chapter Three but don't consummate it till Chapter Twelve--which, incidentally, comes after a five-year time jump. You see, when they first marry she's still grieving her first husband, and then the hero gets sent away on military service in Canada. But it never even occurs to them to seek an annulment, because they know darn well the option doesn't exist.
As a reader, sometimes these and similar errors bother me enough to throw me out of a story, but sometimes they don't. (Of the four, I'm most forgiving of the kilt, because it's not like the plausibility of the whole plot stands and falls on the hero's wardrobe.) If the writer hooks me hard enough, I can make my inner nitpicker sit down and shut up for the duration. Well, not if the error is New World foods in Europe before 1492. The line must be drawn somewhere.
What about you? How tolerant are you of myth-tory in your history?