A document I was testing caused the following warning:

      /usr/bin/onsgmls:<OSFD>0:73:109:3086863860.183:X: reference to non-existent ID "FOO"

As far as I can tell (but can't find much doc on it) X is a type of warning like W and Q, and unlike E which 
are errors. Our algorithm for documents with only warnings is to report a valid result and output the 
warning, e.g http://qa-dev.w3.org/wmvs/HEAD/dev/tests/ampersand-as-data.html

However in this case the result was 
Failed validation, 0 error
Which is, well... wrong.

My knowledge or parse() and parse_errors() did not help me find the cause of this. Any idea? 

Note that the development version 0.8dev (based on S::P::O) did not show this behavior.