Error during registration for multiple participants
The error is due to call to the protected function getRegistrationContactID() in CRM_Event_Form_Registration_AdditionalParticipant class for multiple event participants registration.  

The issue got resolved in my installation (civicrm 4.7.19) after changing the access control of the function getRegistrationContactID() from protected to public.

The file location is civicrm-module/CRM/Event/Form/Registration.php