Paypal payments - Pending (Incomplete Transaction)
Issue had been filed https://issues.civicrm.org/jira/browse/CRM-18483 and this is my fix https://github.com/civicrm/civicrm-core/pull/8270/files 

As earlier in case of drupal site, CRM_Utils_System::loadBootStrap() eventually calls CRM_Utils_System_Drupal::loadBootStrap() and fails at https://github.com/civicrm/civicrm-core/blob/master/CRM/Utils/System/Drupal.php#L545 because we are not passing any drupal userID as $uid parameter in loadBootStrap() fn and got halt at the same point. Unlike in other CMS (wordpress, joomla) the underlying bootstrap code is not too reliant on user id and didn't break. 

As per my fix CRM_Utils_System::loadBootStrap($params, TRUE, FALSE) where 2nd parameter stands for $loadUser = TRUE and third $throwError = FALSE so if there is no corresponding CMS userID assigned against civiCRM contactID then it won't break rather it will return false and will complete the workflow just what happen when we call REST url https://github.com/civicrm/civicrm-core/blob/master/CRM/Utils/REST.php#L711. 

The reason behind calling loadBootstrap function lies in https://issues.civicrm.org/jira/browse/CRM-18245 

Can anyone please verify my patch if it works for ya!!

Thanks