This error results from a call to ./bin/mail
$ ./bin/mail list-outbound [2014-10-08 20:10:43] ERROR 2: Invalid argument supplied for foreach() at [/phabricator/src/infrastructure/events/PhabricatorEventEngine.php:18] #0 PhabricatorEventEngine::initialize() called at [<phabricator>/src/infrastructure/env/PhabricatorEnv.php:115] #1 PhabricatorEnv::initializeCommonEnvironment() called at [<phabricator>/src/infrastructure/env/PhabricatorEnv.php:67] #2 PhabricatorEnv::initializeScriptEnvironment() called at [<phabricator>/scripts/__init_script__.php:21] #3 init_phabricator_script() called at [<phabricator>/scripts/__init_script__.php:24] #4 require_once(string) called at [<phabricator>/scripts/mail/manage_mail.php:5]
The command still returns appropriate results after the error message.
I suspect this has a similar resolution as @T6252, though my PHP skills are lacking.