I listed three things to fix after my theme migration. I added some reasonable text to the about page and found the source of the weird cruft on the last page of posts. It turns out that somehow I had corrupted the text of one of my old posts and substituted the contents of a PNG file. I’m not sure how that happened but the fix was easy: delete it.
Along the way, I got a little more familiar with how Hugo generates pages. Since I can’t easily see what is generated and served on Netlify, it’s nice to be able to run the same command locally and see exactly what is done. Since the contents are are all static, I just looked at the offending page and it was easy to figure out.