Strangely enough, it doesn't cause the build to fail. But the exception is, however, being logged.
It is consistently happening on every build of mediawiki-extensions-hhvm and mediawiki-extensions-zend, including those for MediaWiki core and other extensions.
2015-03-04 18:46:33 lanthanum my_wiki-unittest_: [988f8226] [no req] Flow\Exception\InvalidDataException from line 36 of /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/extensions/Flow/includes/Data/Listener/NotificationListener.php: Invalid metadata for revision scx0gbh8r2a4iw9e #0 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/extensions/Flow/includes/Data/Listener/DeferredInsertLifecycleHandler.php(36): Flow\Data\Listener\NotificationListener->onAfterInsert(Object(Flow\Model\PostRevision), Array, Array) #1 [internal function]: Flow\Data\Listener\{closure}() #2 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/includes/deferred/CallableUpdate.php(27): call_user_func(Object(Closure)) #3 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/includes/deferred/DeferredUpdates.php(105): MWCallableUpdate->doUpdate() #4 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/extensions/Flow/tests/phpunit/PostRevisionTestCase.php(200): DeferredUpdates::doUpdates() #5 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/extensions/Flow/tests/phpunit/Collection/RevisionCollectionPermissionsTest.php(286): Flow\Tests\PostRevisionTestCase->store(Object(Flow\Model\PostRevision)) #6 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/extensions/Flow/tests/phpunit/Collection/RevisionCollectionPermissionsTest.php(164): Flow\Tests\Collection\RevisionCollectionPermissionsTest->generateRevision('new-post', NULL) #7 [internal function]: Flow\Tests\Collection\RevisionCollectionPermissionsTest->testPermissions(Object(User), 'history', Array) #8 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php(988): ReflectionMethod->invokeArgs(Object(Flow\Tests\Collection\RevisionCollectionPermissionsTest), Array) #9 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php(838): PHPUnit_Framework_TestCase->runTest() #10 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestResult.php(648): PHPUnit_Framework_TestCase->runBare() #11 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php(783): PHPUnit_Framework_TestResult->run(Object(Flow\Tests\Collection\RevisionCollectionPermissionsTest)) #12 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/tests/phpunit/MediaWikiTestCase.php(132): PHPUnit_Framework_TestCase->run(Object(PHPUnit_Framework_TestResult)) #13 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(779): MediaWikiTestCase->run(Object(PHPUnit_Framework_TestResult)) #14 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(749): PHPUnit_Framework_TestSuite->runTest(Object(Flow\Tests\Collection\RevisionCollectionPermissionsTest), Object(PHPUnit_Framework_TestResult)) #15 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(709): PHPUnit_Framework_TestSuite->run(Object(PHPUnit_Framework_TestResult), false, Array, Array, false) #16 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(709): PHPUnit_Framework_TestSuite->run(Object(PHPUnit_Framework_TestResult), false, Array, Array, false) #17 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(709): PHPUnit_Framework_TestSuite->run(Object(PHPUnit_Framework_TestResult), false, Array, Array, false) #18 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php(709): PHPUnit_Framework_TestSuite->run(Object(PHPUnit_Framework_TestResult), false, Array, Array, false) #19 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/TextUI/TestRunner.php(350): PHPUnit_Framework_TestSuite->run(Object(PHPUnit_Framework_TestResult), false, Array, Array, false) #20 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/TextUI/Command.php(176): PHPUnit_TextUI_TestRunner->doRun(Object(PHPUnit_Framework_TestSuite), Array) #21 /srv/deployment/integration/phpunit/vendor/phpunit/phpunit/PHPUnit/TextUI/Command.php(129): PHPUnit_TextUI_Command->run(Array, true) #22 /srv/ssd/jenkins-slave/workspace/mediawiki-extensions-zend@2/src/tests/phpunit/phpunit.php(239): PHPUnit_TextUI_Command::main() #23 {main}