When Microsoft revved up its end of life notices for Windows Server 2003, it had no idea that a significant bug was waiting in the wings to squash customer efforts to comply. The bug caused mixed Windows Server 2003 and Windows Server 2012 R2 environments to stop communicating due to Kerberos authentication issues.
Microsoft initially provided a workaround and promised a permanent Hotfix. At the time, the timeline for the release of the Hotfix was questionable, with Microsoft just stating that "it's going to take us some time to get it out to you." That didn't shock too many people, considering it needed to be a functional change.
Well, the problem has now been fixed through a just released Hotfix. The Hotfix is for Windows Server 2012 R2.
P.S. Windows Server 2003 reaches end of life on July 14, 2015.
