I'm often wary of historical books and TV shows, because of the (almost inevitable) inaccuracies. I was recently watching Deutschland 83, a show about which people have justly raved. It has been credited with a Mad Men-like attention to period detail.
So it was rather jarring to see a character with multiple foreign passports, one of which was British, and looked like this:
Anyone who ever had one, knows that UK passports in the 1980s looked like this:
Now it might seem like a trivial error (and maybe most viewers in Germany would not have noticed), but for me this kind of mistake is like the Van Halen Brown M&Ms.
Which is to say, if the producers made a small mistake like this, I start assuming they got other things wrong, things I wouldn't have known about.
It works the same way in books. I was reading a book (which I shan't name), set in England in the 1840s. It seemed well-researched, until the main character spoke of sitting "Indian style", meaning "cross-legged". At that moment, I was reminded that the author was American, and living in the present day, and that many other things in the story were probably also wrong - I just hadn't spotted them.