Domain Controller Upgrade causes Exchange outage

The other day we experienced an exchange outage at a customer site while doing DC upgrades from 2003 to 2008 R2. The exchange box was reporting the error below:

The fix was simply to recycle the Microsoft Exchange Active Directory Topology Service and it's dependents (it was pointing to the DC we were upgrading and thus it went down and the outage occurred). We recycled the service and it pointed to the other healthy DC in the site.