k-fish @ https://github.com/phpbb/phpbb/pull/2232 :

In modules_test.php::test_modules_auth() , p_master::module_auth() calls the static module_auth function, but module_auth references phpbb_dispatcher, which may not have been instantiated. This causes Call to a member function trigger_event() on a non-object as mentioned in https://github.com/phpbb/phpbb/pull/1943

Ran into this while running HHVM's test runner (which can run in parallel). Let me know if a more formal write-up is needed.