One of the examples which has always annoyed me inordinately occurred right at the beginning of that famous 1981 tv adaptaton of Evelyn Waugh's Brideshead Revisited. It started with Captain Ryder saying "Good morning, Sergeant-Major" to a soldier wearing the insignia of a Colour-Sergeant. One can see how the error happened: three stripes on the upper arm are the insignia of a sergeant; the officer ranked major wears a crown on his shoulder. So the unaware might think that a crown above three stripes signify a sergeant-major; but not so: they are a colour-sergeant or staff-sergeant, depending on the regiment. A Company Sergeant-Major [which this one was supposed to be] wears a crown on his forearm.

Important? Well, they spent literally millions on this series, including the services, acc to the credits, of a military adviser; so what was he doing for his money, eh?

~Michael~