Relates to the problem of trying to fallback from a wildcard cert to simple cert, which is currently impossible, as this triggers a rate limit error of too many certificates already issued for exact set of domains... which is wrong...
This results in certs which can never be renewed, if the wildcard cert becomes corrupt or the wildcard client begins failing.
https://community.letsencrypt.org/t/changing-from-wildcard-cert-to-simple-cert-triggers-rate-limit-failure/143237 provides more discussion about this problem.