I pulled this out of the logs; not sure how anyone hit it, but this controller could bump into an undefined variable here:
2015/06/24 22:59:55 [error] 10150#0: *172493 FastCGI sent in stderr: "PHP message: [2015-06-24 22:59:55] EXCEPTION: (RuntimeException) Undefined variable: subscriber_phids at [<phutil>/src/error/PhutilErrorHandler.php:210] PHP message: arcanist(head=master, ref.master=b697a3b80bdc), phabricator(head=redesign-2015, ref.master=0fd0f171f10f, ref.redesign-2015=1a5f986d73dd), phutil(head=master, ref.master=74c9cb3a266e) PHP message: #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/subscriptions/controller/PhabricatorSubscriptionsListController.php:32] PHP message: #1 <#2> PhabricatorSubscriptionsListController::processRequest() called at [<phabricator>/src/aphront/AphrontController.php:33] PHP message: #2 <#2> AphrontController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:226] PHP message: #3 phlog(RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php:229] PHP message: #4 AphrontDefaultApplicationConfiguration::handleException(RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:230] PHP message: #5 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:140] PHP message: #6 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:21]" while reading response header from upstream, client: 167.114.156.198, server: , request: "GET /subscriptions/list/phid-wiki-366842d394398204f305/ HTTP/1.1", upstream: "fastcgi://unix:/core/var/pool/fpm.sock:", host: "secure.phabricator.com"
Clean things up a bit.