Erm. So, WordPress had replaced the widgets in my right sidebar with my penultimate post, but only on the front page. If I clicked a post to view it on its own page, the sidebar worked correctly. But it persisted even if I deleted a post or changed my theme.
I managed to fix it, but I’m not sure how. I do know that my web host upgraded WordPress a few days ago and that I only noticed the issue yesterday. Yesterday I did notice that the html editor was adding a lot of div tags that it wasn’t using before. I went through my last few posts and replaced them with the good ol’ P (paragraph) tag. The front page works again. I’m not 100% convinced it’s because of the deleted div tags, but the sidebar in my theme is indeed differentiated with div tags, so maybe???