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}