Recursive caching servers behavior with lame server answers

[Tony Schenk]

I've been doing snoops to see exactly what information comes back in
packets from "lame" authoratative servers and I'm confused about
something that is probably pretty basic.

In most cases, my caching/recursive name server *knows* the delegation
is lame because the putative server reveals the correct authoritative
server list (which he is not a part of). Why not just follow those NS
records and get the answer? From what I can see(BIND 9.3.1), it repeats
the original request (to the original lame server), gets the same list
of NS records, and returns a SERVFAIL to the downstream requesting name
server.

Any help would be appreciated.