The issue began 2014-04-22.  I wrote about it later on Identi.ca (and Twitter):
* https://identi.ca/ossguy/note/w96IJZNOQRyfN0nYqWqQfA
* https://identi.ca/ossguy/note/nbPznOy9QzabYSEM6d0VWw

OpenDNS then resolved it (on 2014-04-24) with this note:


Some technical detail: Computers that are configured to use both IPv6 and IPv4
will perform a DNS query for both the A and AAAA records for a given domain. Our
resolvers will properly return a NODATA response for the AAAA record, but we are
seeing the client then perform another query for the AAAA record with the local
search suffix attached. This subsequent query will result in an NXDOMAIN
response, and thus customers with NXDOMAIN redirection enabled will be
redirected to our NXDOMAIN page. The A record query will properly return the IP
address, but some clients may have a preference for AAAA records, or may choose
one of the two responses at random, and thus get redirected to our NXDOMAIN
page.

This extra AAAA query is unusual behaviour for a client, but we were able to
mitigate it by removing the redirection for the AAAA record requests from our
DNS servers.