In early February we are going to introduce a Failed Validation limit, on a per-hostname, per-account basis. The production limit will be 5 failures per hour. The staging limit will be 60 per hour. Once the limit is hit, the affected account will not be able to create new authorizations for the affected hostname until the limit is expired. Other hostnames will be unaffected.
Most accounts will not hit this limit.
We’re adding this limit in order to reduce the resource consumption of misconfigured clients that rapidly request authorization and issuance for domain names that no longer exist, or for which the client cannot complete a challenge.