Over on the Fediverse, I mentioned that we once missed noticing that there had been a power failure . Naturally there is a story there (and this is the expanded version of what I said in the Fediverse thread). A necessary disclaimer is that this was all some time ago and I may be mangling or mis-remembering some of the details.
My department is spread across multiple buildings, one of which has my group's offices and our ancient machine room (which I believe has been there since the building burned down and was rebuilt ). But for various reasons, this building doesn't have any of the department's larger meeting rooms. Once upon a time we had a weekly meeting of all the system administrators (and our manager), both my group and all of the Points of Contact , which amounted to a dozen people or so and needed one of the larger meeting rooms, which was of course in a different building than our machine room.
As I was sitting in the meeting room during one weekly meeting, fiddling around, I tried to get my Linux laptop on either our wireless network or our wired laptop network (it's been long enough that I can't remember which). This was back in the days when networking on Linux laptops wasn't a 100% reliable thing, especially wireless, so I initially assumed that my inability to get on the network was the fault of my laptop and its software. Only after a bit of time and also failing on both wired and wireless networking did I ask to see if anyone else (with a more trustworthy laptop) could get on the network. As a ripple of "no, not me" spread around the room, we realized that something was wrong.
(This was in the days before smartphones were pervasive, and also it must have been before the university-wide wireless network was available in that meeting room.)
What was wrong turned out to be a short power failure that had been isolated to the building that our machine room was in. Had people been in their offices, the problem would have been immediately obvious; we'd have seen all networking fail, and the people in the building would have seen the lights go out and so on. But because the power issue hit at exactly the time that we were all in our weekly meeting in a different building, we missed it.
(My memory is that by the time we'd reached the machine room the power was coming back, but obviously we had a variety of work to do to clean the situation up so that was it for the meeting.)
For extra irony, the building we were meeting in was right next to our machine room's building, and the meeting room had a window that literally looked across the alleyway at our building. At least that made it quick and easy to get to the machine room, because we could just walk across the bridge that connects the two buildings.
PS: In our environment, this is such a rare collection of factors that it's not worth trying to set up some sort of alerting for it, especially today in a world with pervasive smartphones (where people outside the meeting room can easily send some of us messages, even with the network down).
(Also, these days we don't normally have such big meetings any more and if we did, they'd be virtual meetings and we'd definitely notice bits of the network going down, one way or another.)