We recently hit an issue with one of our clients where an external service was trying to authenticate against Salesforce with oauth 2.0 user-pass flow but kept receiving "unknown_error.retry your request" from Salesforce. We searched online and used #askforce but the only fix we found was to loging using the Salesforce instance, which does not work for us as we are trying to access Apex REST in a managed package that will be available on the AppExchange sometime soon.
So we opened a case through the partner portal which was quickly
confirmed and escalated. Within a week, we receive a response that Tier 3 have identified the issue and that R&D is working on a patch that will likely to be released today (19th Dec 2012). They did not elaborate much further even though we prodded for some more geeky details, except to say that "the issue was due to internal issue" and that "the patch will be released on all instances".
Kudos to Salesforce for a quick turnaround!