Big fan of pop music, mostly secular, from 1300s-1850s. Studying Bach manuscripts directly (or repros, as I had to settle for) indicated to me that publishers did a lot of guesswork over the years, some of it obviously wrong, but a bunch more of it just their best guess. I decided I was just ignorant about the number of errors I saw.

I managed to score a lunch with Yo-Yo Ma at a charity event a few years ago & was proven satisfyingly right. He had to make a ton judgment calls but found that over the years many errors have been introduced by publishers. Of course he was able to study the originals.

Also the British Museum will give almost anyone with a passport surprisingly liberal access to their collections. Traveled there from the US with my teenage kid only to find out the "almost" part, which is that you must be 18 or older. Understandable, but... sigh.