Not too often would you need this, but I just ran into a situation where it was handy to get AD off an existing, but inactive, domain controller.  I couldn’t get dcpromo to uninstall the services and demote the darn thing, because this server thought it was still active in the domain.  I had physically removed this server and manually cleared it from the active domain.  Why would you do this, well, that’s a long story, but I wanted to, let’s just say that.

Here’s how you demote a DC with dcpromo, forcefully.

“dcpromo /forceremoval”

I did this on a 2008 Server, worked great!  All these years and I hadn’t needed to do this, and didn’t even know it could be done.  Always something new to learn, huh!