I just got the 503 Backend fetch failed when posting a chapter, but I knew the chapter might have posted anyway, so I went and loaded the main page in another tab and it was there. No double post for me!

It has something to do with the way posting on web sites work. The post and the page load are two separate actions. The post uploads your content and the web site makes the page. It's possible for the web site to send back your page content in the response, but instead it sends back a redirect, a message that just tells your browser to load the page where the content was posted. Sending the browser to a non-post page avoids double-posting if your browser reloads that page. But it also invites the possibility that the post works and the page load afterward fails, like happened to both of us, and then you might end up double-posting because you think your post didn't work.