Confusing BIND with CNAMEs
Given the zone:
subdomain IN NS nameserver nameserver IN CNAME nameserver.otherserver
in a BIND server that is both recursive and authoritative, requests without RD (recursion desired) return the NS record, while requests with RD return SERVFAIL. Changing it to:
subdomain IN NS nameserver.otherserver
fixes the problem, though both resolution trees are perfectly valid.