TIL that #letsencrypt has a rate limit for failed authorizations. :)
An unexpected error occurred:
too many failed authorizations (5)
https://letsencrypt.org/docs/rate-limits/#authorization-failures-per-identifier-per-account

Rate Limits
Let’s Encrypt provides rate limits to ensure fair usage by as many people as possible. We believe these rate limits are high enough to work for most people by default. We’ve also designed them so that renewing a certificate almost never hits a rate limit, and so that large organizations can gradually increase the number of certificates they can issue without requiring intervention from Let’s Encrypt. If you’re actively developing or testing a Let’s Encrypt client, please utilize our staging environment instead of the production API. If you’re working on integrating Let’s Encrypt as a provider or with a large website please review our Integration Guide.