I don't know the answers to this, but here are a couple of things I have run into.
For the novel I am revising now (working title Death on the Delaware) I have to deal with the change from the Julian calendar to the Gregorian calendar. I researched it when I started the story a couple of years ago. I did it again before I picked it up for the first uncompleted rewrite. Third time, I am now looking into it again to participate in a words-per-day writing challenge. First time I didn't find it interesting enough to incorporate it into the writing, except by mere mention. Second time I discovered the "Give us our 11 days" campaign. Nice etching by Hogarth. That would be fun to write about. So I shifted the dates a bit, only to abandon the work again. Third time, and it may stick. Turns out (maybe) the GUOED campaign may have been a figment of Hogarth's imagination. There seem to be only two references to it, Hogarht's picture and one other.
Emily and Charles (The Case Book of Emily Lawrence) attend three churches in the course of the book: First Parish Cambridge, what is now All Souls in Washington, and The Universalist church in Rutland, Vermont. All Souls has a website that lists many of the ministers but not the one who served while E&C were attending. They built a new building while my characters attended, but the present building is newer still. First Parish has been helpful in several ways. I wrote the whole first Emily novel with the idea that the church interior had never changed. That is a pretty dumb assumption. Ernest Cassara, board chair, showed me a photograph of the interior as Emily would have seen it. The current minister, Clyde Grubs, put me in touch with the Andover Harvard librarian who gave me the name of the minster who had married the happy couple.
I didn’t need as many details for Rutland, but the church historian supplied the name of the cemetery where Charles and his family are buried, and a few facts about the church itself. My first go round made me feel secure that the church was there, so I wrote it into the story. It turned out it wasn't established until after I need it, so I made the assumption that a group of funders met for some time in homes until they pulled together enough support to establish he church.
I suspect I read what I want to believe the first time around and then when I recheck, I begin to see the light.