You're the second person in these comments who points out faux name server redundancy. Interesting. I hadn't considered it as a major theme, but I'll move it up the list a little.

> Authoritative drift.

This is why I query all authoritative name servers (as well as delegating name servers when querying NS records) in the DNS propagation tool. I haven't seen any other site do this. This feels like such an obvious thing to me, but somehow I'm the only one.