A Team Foundation Server instance I manage, on TFS 2012 Update 1, has had some unhealthy signs for a little while – notifications stopped working, and viewing history for some files\branches displayed an ominous error:
It turned out to be due to Active Directory accounts for some contractors being deleted. Microsoft has a hotfix for this, but in our case updating to TFS 2012 Update 4 did the trick.
Side effect: TFS sent me 350+ notification emails that had been queued up and unable to send. Whoops!