Just looking through our PHP error logs and came across this, sorry that I can't provide reproduction steps:
[03-Feb-2016 09:05:52 Australia/Sydney] [2016-02-03 09:05:52] EXCEPTION: (AphrontDuplicateKeyQueryException) #1062: Duplicate entry 'PHID-TASK-joohb6xcsyhhq3upga2v-PHID-USER-hfenohlvucsbthy2su73-0' for key 'key_object' at [<phutil>/src/aphront/storage/connection/mysql/AphrontBaseMySQLDatabaseConnection.php:300] [03-Feb-2016 09:05:52 Australia/Sydney] arcanist(head=master, ref.master=57f6fb59d739), phabricator(head=master, ref.master=d41aaba2a184, custom=1), phlab(head=master, ref.master=450c7c100a72), phutil(head=master, ref.master=f43291e99d36) [03-Feb-2016 09:05:52 Australia/Sydney] #0 <#2> AphrontBaseMySQLDatabaseConnection::throwCommonException(integer, string) called at [<phutil>/src/aphront/storage/connection/mysql/AphrontBaseMySQLDatabaseConnection.php:333] [03-Feb-2016 09:05:52 Australia/Sydney] #1 <#2> AphrontBaseMySQLDatabaseConnection::throwQueryCodeException(integer, string) called at [<phutil>/src/aphront/storage/connection/mysql/AphrontBaseMySQLDatabaseConnection.php:275] [03-Feb-2016 09:05:52 Australia/Sydney] #2 <#2> AphrontBaseMySQLDatabaseConnection::throwQueryException(mysqli) called at [<phutil>/src/aphront/storage/connection/mysql/AphrontBaseMySQLDatabaseConnection.php:181] [03-Feb-2016 09:05:52 Australia/Sydney] #3 <#2> AphrontBaseMySQLDatabaseConnection::executeRawQuery(string) called at [<phutil>/src/xsprintf/queryfx.php:6] [03-Feb-2016 09:05:52 Australia/Sydney] #4 <#2> queryfx(AphrontMySQLiDatabaseConnection, string, string, string, array, string) [03-Feb-2016 09:05:52 Australia/Sydney] #5 <#2> call_user_func_array(string, array) called at [<phutil>/src/aphront/storage/connection/AphrontDatabaseConnection.php:28] [03-Feb-2016 09:05:52 Australia/Sydney] #6 <#2> AphrontDatabaseConnection::query(string, string, string, array, string) called at [<phabricator>/src/infrastructure/storage/lisk/LiskDAO.php:1261] [03-Feb-2016 09:05:52 Australia/Sydney] #7 <#2> LiskDAO::insertRecordIntoDatabase(string) called at [<phabricator>/src/infrastructure/storage/lisk/LiskDAO.php:1106] [03-Feb-2016 09:05:52 Australia/Sydney] #8 <#2> LiskDAO::insert() called at [<phabricator>/src/infrastructure/storage/lisk/LiskDAO.php:1075] [03-Feb-2016 09:05:52 Australia/Sydney] #9 <#2> LiskDAO::save() called at [<phabricator>/src/applications/draft/storage/PhabricatorVersionedDraft.php:62] [03-Feb-2016 09:05:52 Australia/Sydney] #10 <#2> PhabricatorVersionedDraft::loadOrCreateDraft(string, string, NULL) called at [<phabricator>/src/applications/transactions/editengine/PhabricatorEditEngine.php:1469] [03-Feb-2016 09:05:52 Australia/Sydney] #11 <#2> PhabricatorEditEngine::buildCommentResponse(ManiphestTask) called at [<phabricator>/src/applications/transactions/editengine/PhabricatorEditEngine.php:790] [03-Feb-2016 09:05:52 Australia/Sydney] #12 <#2> PhabricatorEditEngine::buildResponse() called at [<phabricator>/src/applications/maniphest/controller/ManiphestTaskEditController.php:12] [03-Feb-2016 09:05:52 Australia/Sydney] #13 <#2> ManiphestTaskEditController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:237] [03-Feb-2016 09:05:52 Australia/Sydney] #14 phlog(AphrontDuplicateKeyQueryException) called at [<phabricator>/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php:27] [03-Feb-2016 09:05:52 Australia/Sydney] #15 PhabricatorAjaxRequestExceptionHandler::handleRequestException(AphrontRequest, AphrontDuplicateKeyQueryException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:632] [03-Feb-2016 09:05:52 Australia/Sydney] #16 AphrontApplicationConfiguration::handleException(AphrontDuplicateKeyQueryException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:242] [03-Feb-2016 09:05:52 Australia/Sydney] #17 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:149] [03-Feb-2016 09:05:52 Australia/Sydney] #18 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:17] ``