No ... you are not going crazy. You just joined a Windows 10 Pro machine (that you in place upgraded from Home likely) to a Windows Server 2012 R2 Essentials domain. Joined just fine. Reboot. Try and login: Wrong username or password. 

If you are like me you try several known-good username / password combos lot's of times until you start to doubt your sanity. 

Silly you! It's Windows 10!!!

Here is the best thread I've found about the issue: 

The fix for me: Run the Windows 10 Media Creation Tool under a local admin account, "upgrade", reboot. Viola.