intertwingly

curl -i -d "<rss><channel/></rss>" http://feeds.archive.org/validator/
curl -i -d "<rss/>" http://feeds.archive.org/validator/

The latter demonstrates a bug in the validator, which I will leave unfixed for the moment to demonstrate fault responses (complete with server side python stack traceback).  This can be put into a script on Unix/Windows thus:

curl -i -d "`cat $1`" http://feeds.archive.org/validator/
curl -i -d "@%1" http://feeds.archive.org/validator/

Thanks go out to Jeffrey Winter for this nugget.