I saw this error (a few times so far) while building wikidata's index:

[04-Dec-2013 22:41:54] Catchable fatal error: Argument 1 passed to CirrusSearchTextFormatter::formatWikitext() must be an instance of ParserOutput, boolean given, called in /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchUpdater.php on line 466 and defined at /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchTextFormatter.php on line 27
Server: mw1009
URL: http://[unknown-host]
Backtrace:
#0 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchTextFormatter.php(27): CirrusSearchTextFormatter::formatWikitext()
#1 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchUpdater.php(466): CirrusSearchTextFormatter::formatWikitext(false)
#2 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchUpdater.php(329): CirrusSearchUpdater::buildTextToIndex(Object(WikitextContent), false)
#3 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchUpdater.php(150): CirrusSearchUpdater::buildDocumentForPages(Array, 0)
#4 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchUpdater.php(83): CirrusSearchUpdater::updatePages(Array, false, '1ms', 5, 0)
#5 /usr/local/apache/common-local/php-1.23wmf5/extensions/CirrusSearch/includes/CirrusSearchLinksUpdateJob.php(34): CirrusSearchUpdater::updateFromTitle(Object(Title))
#6 /usr/local/apache/common-local/php-1.23wmf5/maintenance/runJobs.php(101): CirrusSearchLinksUpdateJob->run()
#7 /usr/local/apache/common-local/php-1.23wmf5/maintenance/doMaintenance.php(113): RunJobs->execute()
#8 /usr/local/apache/common-local/php-1.23wmf5/maintenance/runJobs.php(181): require_once('/usr/local/apac...')
#9 /usr/local/apache/common-local/multiversion/MWScript.php(97): require_once('/usr/local/apac...')
#10 {main}

Version: unspecified
Severity: major