Hey,
This may be a known, but just in case it isn't; the pageview dumps at http://dumps.wikimedia.org/other/pagecounts-all-sites/ are meant to follow the spec set out at http://dumps.wikimedia.org/other/pagecounts-all-sites/README.txt
Instead, it appears that for (presumably, zero-rated) requests, we're ending up with lang_code.zero instead of lang_code.project_variant. Presumably it's a missed use case in the C/Perl...thing, we were using, that got ported to Hive? Check out pagecounts-20150301-000000 for an example.
I've opened a phabricator ticket at https://phabricator.wikimedia.org/T92361 - this is just an advisory to analytics engineers (there is a bug) and to reusers (there is a bug. We're aware of the bug).
Have fun,