Found in logstash after irc user Raymond_ reported seeing a fatal error on his Commons watchlist page:
2014-03-25T20:06:45.000Z Invalid option hideWikibase
#0 /usr/local/apache/common-local/php-1.23wmf19/includes/FormOptions.php(176): FormOptions->validateName('hideWikibase', true)
#1 /usr/local/apache/common-local/php-1.23wmf19/extensions/Wikidata/extensions/Wikibase/client/includes/hooks/SpecialWatchlistQueryHandler.php(49): FormOptions->getValue('hideWikibase')
#2 /usr/local/apache/common-local/php-1.23wmf19/extensions/Wikidata/extensions/Wikibase/client/WikibaseClient.hooks.php(318): Wikibase\Client\Hooks\SpecialWatchlistQueryHandler->addWikibaseConditions(Object(WebRequest), Array, Object(FormOptions))
#3 [internal function]: Wikibase\ClientHooks::onSpecialWatchlistQuery(Array, Array, Array, Array, Object(FormOptions))
2014-03-25T20:00:07.000Z PHP Fatal error: Call to a member function getValue() on a non-object in /usr/local/apache/common-local/php-1.23wmf19/extensions/Wikidata/extensions/Wikibase/client/includes/hooks/SpecialWatchlistQueryHandler.php on line 49
Version: unspecified
Severity: blocker