Reconnecting a long before dissconnected DC
Just today had a situation where we had some strange behaviour. Some users and OUs were not replicating. OK I go in do some diagnostics just to find out those are some two old DCs which had been brought back to life after like 4 months.
The thing is their Tombstone time expired and now they were no good (if you know what I mean). It took some time and a whole lot of searching 
Here is the TechNet article on it.
Reconnecting a Domain Controller After a Long-Term Disconnection
P.S. I passed 70-298 my finall exam for the MCSE: Security
