civicrm/civicrm-core — pull request #3292 by eileenmcnaughton (2014-05-19 08:50:08+00:00)
Title: CRM-14708 accept POSTs without x_cust_id based on missing contact_id … … 

I think this is actually non-controversial as the 2 out of 3 logic is already in place it's just that it's aborting on blank contact id AND the code setting $ids['contact'] is referring to $ids['contact_id'] - but in face $ids['contact'] appears to be correct