It was the bad old days of sysadmin, where literally every critical service ran on an iron box in the basement.

I was on my first oncall rotation. Got my first call from helpdesk, exchange was down, it's 3AM, and the oncall backup and Exchange SMEs weren't responding to pages.

Now I knew Exchange well enough, but I was new to this role and this architecture. I knew the system was clustered, so I quickly pulled the documentation and logged into the cluster manager.

I reviewed the docs several times, we had Exchange server 1 named something thoughtful like exh-001 and server 2 named exh-002 or something.

Well, I'd reviewed the docs and helpdesk and stakeholders were desperate to move forward, so I initiated a failover from clustered mode with 001 as the primary, instead to unclustered mode pointing directly to server 10.x.x.xx2

What's that you ask? Why did I suddenly switch to the IP address rather than the DNS name? Well that's how the servers were registered in the cluster manager. Nothing to worry about.

Well... Anyone want to guess which DNS name 10.x.x.xx2 was registered to?

Yeah. Not exh-002. For some crazy legacy reason the DNS names had been remapped in the distant past.

So anyway that's how I made a 15 minute outage into a 5 hour one.

On the plus side, I learned a lot and didn't get fired.