Fatal error: Detected non-empty blocksets with no associated blocks

I missed this post saying now a repair fixes this problem. I did not try a repair.

I don’t understand the internals enough. There’s at least three entities: database, blocksets, and blocks. And dlists? and indexes?

I’m not sure what this error is indicating is broken, and what the various repairs do. Is there one I should try? I’ll try to read up on this when I have time.