I started reading a story that is an updated version of a story written in the distant past. I've only gotten through the first six chapters or so. In doing so I've encountered formatting errors that make reading the story difficult. So I thought about advising the author of the issue, but... not sure how to do it.
For the stories I've written, I always opened them after they were posted to see if I screwed up.
What I've done with other stories that I liked but had formatting issues, I waited until it was complete. Then after downloading the ePub, I edited it to fix the formatting errors.