[Bender]

Adding to this I would take it a step further. One should use monitoring probes from many locations that check the authoritative servers directly in the event one is using anycast DNS (hundreds or thousands of DNS servers spread around the world) to increase the likelihood that one catches a cluster out of sync SOA mismatch or a cluster is down or experiencing packet loss. There are services that do this or one could just use in-house monitoring from all their data-centers in each continent and region. Even and especially in the biggest DNS providers anycast clusters can get out of sync for a myriad of reasons which is why the probe should always be for the SOA serial of each zone.