I write in the Georgian era, and here are a few of the mistakes commonly made. Plots that I've seen that either need a bit more work to make plausible or wouldn't work at all.
1. The duke marrying the governess. No. Just no. A gentleman, a high-ranking member of the upper middle class, yes, but if a duke marries a governess, he can expect to be socially shunned.
2. Speaking of which - disregarding the fact that society will shun you. Better than imagining it's no problem. This didn't mean not being invited to a few parties, it meant being cut off from everything that made the peerage what it was. Being an aristocrat in this era was similar to being the Chairman of the Board or Senior Executive in a big-ass corporation these days. Not exact, but near enough for the analogy to work. Being cut off meant having your peerage disbarred. 'Companies,' that is, other peers, the network of financial organisation, contacts and goodwil that make a company work, all gone. So yes, it happened, but it also could lead to the total destruction of the 'company' or peerage, and all the structures that depended on it for their living. Estate workers, farm workers, lawyers, servants, industries - everything.
If a man's word couldn't be trusted, then the structure collapsed, too. The code of honour meant something.
3. Another situation - the hero and heroine blithely assuming they could have a 'temporary' marriage, that they could divorce or have the marriage annulled after a trial period. Never, ever. Divorces involved an Act of Parliament, and wherever the fault lay, usually put the woman beyond the pale. Annulments were so rare as to be discounted, and when they did occur, they were for legitimate reasons - and those reasons were rigorously tested. Again, yes, it happened, rarely, but the consequences were dire, especially for the ex-wife.
Consummation has never been valid grounds for an annulment of a marriage. There isn't one case of a marriage annulled from non-consummation in the Georgian era. An annulment on the grounds of the male's impotence could be invoked, but the male's impotence had to be tested, by putting him in a room with several sexy women who would try to arouse him. One doubtful case in the late Georgian era is all we've been able to find. But no, annulment for non-consummation never existed in the Georgian era and wasn't valid grounds.
4. The duke (or marquess or earl) marrying a courtesan and society forgiving and forgetting her notorious past. Never, ever happened. Once a courtesan, always a courtesan. If a peer did something that foolish, then not only him but his children would be tainted. Not to say it didn't happen in a more discreet fashion (I used this loophole in "A Chance To Dream"). But no, such a woman would never, ever be openly acknowledged or accepted in the fashionable salons which were the powerhouses of the time.
5. A woman dressing in bifurcated garments, under or over her clothes. Until the Victorian era, no bloomers or knickers or panties (except for titillation). From a practical pov, imagine trying to pee in one of the primitive toilets or chamber pots of the time, holding voluminous skirts out of the way and trying to hold a pair of panties down as well? And if a woman dressed in male clothing, or rode astride, she would probably be locked away as a lunatic. Menfolk could and did get rid of inconvenient females that way.
6. Women who refuse to marry a man after she has slept with him, on the grounds that "he didn't say he loved her." After she'd lost her virginity, she could well be pregnant and to deny a child the chance of legitimacy carried severe, and permanent consequences for the child. Not the act of a heroine, in my book.
See what I mean? These things could happen, but you have to follow through on the consequences. You can't pick and choose, you have to accept the times as they were.
And what is most frustrating to me is that there are some great stories to be told if the consequences are followed. What happens when your mother is shunned as a Fallen Woman? Do you stick with her, or do you accept the offer of your stiff and proper Auntie Honoria, for her to take you into her household, bring you out into society and find you a husband? Can you turn your back on your much beloved mother? But no, many writers assume that society was as flexible then as it is now, that its mechanism is much the same. It isn't, and it wasn't.
Another reason why I admire people who write in past ages, and who recreate a society long gone. I only write about times 300 years ago, but already it's alien to many readers.
And don't expect your editor to pick up your historical inaccuracies. Editors don't pick up those errors. They aren't there for that, and usually they will question a few points, but editors aren't often history experts, too. They might be editing a variety of books, from paranormals to sweet Inspirationals, to cowboy romances, and they aren't experts on that, and most publishing houses don't expect them to be.