I remember investigating this once before and I think I found that it's a race condition: whenever two requests go in for the same book simultaneously, one of them 500s when it's time to write the generated feed to the database.

This isn't a serious problem on its own, but it clutters the error logs with semi-spurious 500 errors, making it difficult to find more serious errors that crop up.

I imagine this happens whenever two people enter the detail view for the same book–usually because it is featured on the front page.