Seen at http://en.m.wikipedia.beta.wmflabs.org/w/index.php?search=Test+is+used+by+Selenium+web+driver&fulltext=search (at least)
PHP fatal error in /srv/common-local/php-master/extensions/CirrusSearch/includes/InterwikiSearcher.php line 79:
Class 'CirrusSearch\InterwikiResultsType' not found
Version: unspecified
Severity: normal