Probably the single most common historical/factual era I see in romances set in the British 18th or 19th century is botching the noble characters' titles. I ran across this just today in a book that shall remain nameless, one that I'd been looking forward to for MONTHS because of blogosphere buzz, including some interviews with the author that made it clear she had an exciting story concept and had put a lot of work into it.
I've seen a variety of erroneous title usages in my reading, but most of them boil down to writers not seeming to get that you don't just stick "Lord" or "Lady" in front of any part of a character's name. There is a specific correct form depending on who the character is. E.g., Dorothy Sayers' wonderful detective, the younger son of a duke, is Lord Peter, or Lord Peter Wimsey in cases where specificity is called for. He is never, NEVER, Lord Wimsey. Sayers, of course, gets this right. Lots of modern romance authors don't. Lord Peter's nephew--Gerald Wimsey, Viscount Saint-George--is Lord Saint-George, NEVER Lord Gerald or Lord Wimsey. I won't go into the whys and wherefores. Suffice it to say that the correct forms of address are READILY available on the web. Jo Beverley has a nice straightforward guide on her website. Googling "forms address british nobility" gave me this and this, just for starters.
In other words, while the usage is confusing and obscure to the American ear, it's not even remotely tough to find. And so even though I know it's not important, I have little patience with authors who get it wrong. I mean, you wrote a whole novel and you couldn't spend five seconds coming up with a decent Google query and half an hour reading the results it turned up? Why do authors keep missing this? And am I the only one crotchety and pedantic enough to care?