Feb 2 08:45:31 mw1278: #012Notice: Undefined index: HASH in /srv/mediawiki/php-1.29.0-wmf.10/includes/cache/MessageCache.php on line 594 Feb 2 08:45:31 mw1278: #012Notice: Undefined index: HASH in /srv/mediawiki/php-1.29.0-wmf.10/includes/cache/MessageCache.php on line 727
To reproduce, edit or purge a MediaWiki-namespace page for a language that isn't already loaded, e.g. [[MediaWiki:mainpage/en-gb]] if that's not your user language.
The cause seems to be rMWc962b480568e: Avoid races in MessageCache::replace(), in particular the move of the ->load() call from line 567 into a DeferredUpdate that's going to be run later in the request.