bind-9.8.1: INSIST(! dns_rdataset _isassociated(sigrdataset)) failed

[nicku]

To my surprise, I had several DNS servers running BIND 9.8.1 all fail
at about the same time with this assertion failure in query.c, on line
1895.  The only references I have found to this were from
CVE-2009-0696 Dynamic Update DoS attack, which 9.8.1 surely should be
immune to.  Any suggestions as to possible cause are most welcome.