Around the same time icinga-wm issued alerts for failing ElasticSearch shards, this message started appearing in fluorine:/a/mw-log/fatal.log:
2015-06-15 07:00:13 mw1124 enwiki fatal INFO: [ba4a0279] /w/api.php?action=opensearch&format=json&search=REDACTED&namespace=0&limit=10&suggest= ErrorException from line 264 of /srv/mediawiki/php-1.26wmf9/includes/exception/MWExceptionHandler.php: Fatal Error: Class undefined: CirrusSearch\Status MediaWiki or an installed extension requires this class but it is not embedded directly in MediaWiki's git repository and must be installed separately by the end user. Please see <a href="https://www.mediawiki.org/wiki/Download_from_Git#Fetch_external_libraries">mediawiki.org</a> for help on installing the required components. #0 (): MWExceptionHandler::handleFatalError() #1 {main} {"private":false}