CirrusSearch is logging between 20 and 80 errors per minute:
Warning: Invalid operand type was used: expecting an array in /srv/mediawiki/php-1.28.0-wmf.23/extensions/CirrusSearch/includes/Searcher.php on line 588
Those errors started and 00:10 UTC October 28 : https://logstash.wikimedia.org/goto/1cca914e4674692f91b3ea8172096d22