"The server's down!" (yes, singular).
I was working consultancy for individual schools and worked 3-6 hours for a different one, often a few a day.
This was an emergency call while I was at a different customer's site. They were increasingly desperate, which is a good time to talk terms. I agreed I'd come and fix whatever the problem was - however long it took - if they paid emergency rates so that I could justify tearing myself away from the other customer I was already with that day. But also that's all I would be doing - fixing that problem.
Asked the customer I was with and they were happy for me to "swap days" (because it would similarly benefit them if anything were to happen, and it was a rare occurrence).
CYCLED over to the other side of town through freezing ice and snow.
Got there and went to where the server was (it was sited in the school office, sitting in a corner, with its screen turned off - this was "normal" for Borough installs that I had inherited at the time).
Turned the monitor on for the server (it was always switched off to "stop people playing with it" - high security!).
"Press Enter to continue boot...." white DOS text on a black screen. Nothing else there.
I pressed Enter. The machine booted. Everything started working.
Apparently it was a BIOS option that the Borough put on their servers - most ridiculous!
So technically the fix was within a few seconds or so of arriving.
But, diligent as I was, I diagnosed further.
I noticed a disconnected fan heater hidden under the office desk just a few metres away, clearly trying to obscure itself under some cardboard. It was hot to the touch, but not plugged in. I put my Poirot detective skills to the test. There was nowhere else to plug it in except near the server.
I gathered the suspects and had my moment:
The office secretary was cold (it was snowing outside). She wanted to plug in a heater. There were no sockets available. So she plugged it into the extension lead (I know, I know, don't go there, it wasn't my setup!) that the server was plugged into. It popped the breaker. The server went off. Rather than admit to this, she unplugged the heater, hid it, and flipped the breaker back on (the breakers popping happened a lot in that school, especially in winter). The power came back on, the server started to boot, but the BIOS option made it wait forever until someone came along and pressed Enter.
For several seconds of fixing, and 5 minutes detective work, I was paid a full day's wage at emergency rates, and was home by 9am.