It's up there with "unless you test power supply failover, don't rely on that generator to save you".
I know of one case where power failed, the "24 hour" generators kicked in, power company says "We'll need 12 hours" and the outage happened 6 hours later when they ran out of oil.
I contracted at one place for a bit where they shut everything down in each of their data centres once a year and power everything back up.
When I was there this didn't go too well and they couldn't get one their data centres online again - failover to their other centres did work though. This was ~20 years ago in the finance sector.
Yup. Have the problem when all the right people are awake and on-site to handle it.
I was in a building when someone inadvertently powered off the wrong equipment, which had been running for several years, and several of the power supplies failed to come back up. It was 1+1 redundant though, so we could quickly shuffle packs around to bring it back up without redundancy. Then, jogging through the building and asking if anyone had spares, we found a field tech in the lunchroom who had a pile of stuff in his van. Whole thing was back to 100% in less than an hour, and we let the beancounters sort out the field spares being used for office equipment.
If that same failure had happened during the overnight maintenance window (when volatile work was supposed to be performed), there certainly wouldn't have been the same resources around.
I know of one case where power failed, the "24 hour" generators kicked in, power company says "We'll need 12 hours" and the outage happened 6 hours later when they ran out of oil.