Mine was my first real hands on experience with Active Directory and Group Policy..
We had a massive server failure and lost everything on a Thursday evening. Friday was spent troubleshooting and trying to get the server back up and running but eventually found a mass data loss from a bad RAID card that fried just about everything. Friday evening we got a new server and installed Server 2008 R2 - which ended up failing miserably after some OS Updates.. thanks Microsoft. After the OS Failure we installed Server 2012 and I got to work recreating and learning as I went through the hells of getting an entire domain up and running, all systems reconnected and authenticated and all users/usergroups/security groups created.
It was about a 60 hour shift with a few naps at my desk in my office.. but on Monday morning, when I happened to be off, I was able to get everyone their new logins and we were back up and running - minus some data of course.
Other than that.. I used to work for GeekSquad.. every day consisted of Tech Nightmares but the worst nightmares there were generally the people. Nobody likes a pissed off person throwing their camera, phone, laptop or stereo at them...
So what's your biggest tech nightmare?
-Adam