A few days ago I posted about authors, published and otherwise, botching the usage of their noble British characters' titles and wondering why they don't take the time to look it up. I think I've finally figured it out--the writers think that Lord and Lady are the equivalent of honorifics like Mr./Miss/Mrs./Ms./Dr., which at least in modern usage are more flexible. E.g. you might address a professor as "Dr. Smith," but if you were introducing her as a speaker, you'd ask the audience to welcome "Dr. Jane Smith"--the Dr. can go in front of either her last name or first name, depending on the needs of the situation.
Anyway, I think writers are just assuming that Lord and Lady can be used the same way and not even realizing there are different rules that they need to research. Or they may have used published books that get it wrong as a guideline, or even been confused by books that get it right. Someone might read Gaudy Night, see Lord Peter and his nephew Lord Saint-George, and instead of seeing two different forms of address based on the subtle differences in the characters' positions, just think that "Lord" can go in front of any part of a character's name or title.
I'm feeling more charitable toward authors who make these errors now, because I can see how it's often an honest mistake. I'm not sure I would've known to pay attention to it myself if I hadn't seen erroneous forms of address in an article on common errors in Regency romances on an author website long ago. (That said, it still grates on me as I read--I can't help mentally correcting as I go along, which gets wearying.)
In general, I think the trickiest issues are the ones where we don't even know we need to research. There's so much false conventional wisdom out there, so many historical urban legends even. But how does a writer know to question what she has read in so many places that she takes it for granted, unless she just happens to stumble across a debunking of bad history or an explanation of how things really worked? I'm probably making the same kind of errors myself in areas I don't realize I need to research. I'm not normally one to borrow a phrase from Donald Rumsfeld, but it's those unknown unknowns that'll get you every time.