The "Wilhelm de Friess" article has reached that stage where the meat of the argument is completely covered in scar tissue. When I was a couple pages away from wrapping up the first draft, I started adding notes in square brackets in various places, reminding myself to add something important. Then I added a few more notes. Then a few more. Now I have successfully reduced a budding article to a pile of notes and references.
This was not unexpected, actually. It's more or less what happened when I wrote my Copernicus article. It's a useful part of the writing process, because it helps me identify those areas where I no longer support the article's original narrative, and it shows me what open questions still remain at this point. I find that I need to send a couple inquiries to a few libraries and archives to tie up loose ends.
The coolest thing that you can do every day as a professor is to request books from anywhere in the world through interlibrary loan - and get them. The coolest thing that you can do occasionally, though, is to write to librarians and archivists anywhere in the world with questions about obscure topics - and get answers.