"A botched software update at Spain's central domain registry knocked as many as 400,000 sites offline for several hours Tuesday, according to the Esnic registry. The error left Internet users unable to access domains using .es, the country code top-level domain for Spain. The outage lasted from 3 pm to 5 pm Tuesday afternoon (local time) when the DNS database that connects domain names to IP addresses was 'affected by an error' during a software update, according to an advisory posted at Esnic, which manages the .es database."
Perils of DNS. Umm... thats why you have multiple DNS servers so if one server goes down, its not lights out for 400,000 web sites! Upgrade one server, make sure it works, then the other!