Back in the mid 00's I was a pre-sales tech support at a security equipment distributor; my realm being the nascent world of IP CCTV.
One of my customers was working on a system with a hefty non-disclosure agreement attached, so I couldn't know the end user, or the details of how the system would be installed or used; I only had the basic details of the number of cameras, recording rates etc. From this I had to spec the type of cameras, recording servers, storage, managed PoE switches and a UPS to keep the system alive for 6 hours.
I queried the UPS, suggesting a smaller UPS and generator, but the end user had insisted on using a bank of UPSes.
The system was duly specced, installed and commissioned by my customer and I heard nothing else for 6 months until I got a support call on Monday morning. The customer had experienced a long powercut over the weekend, all was good, the recording servers had run for the full 5 hours the power was down, but they hadn't recorded anything.
My forst though was the cameras, which were all indoors and this not fitted with infra-red illuminators, had recorded, but you couldn't see anything because the lights were out.
No, the guy says, the site had emergency lighting, so there should be footage.
As his customer was important to him, my customer paid to fly me to site; which turned out to be a large casino, gentleman's club and a slightly more exclusive gentleman's establishment in Berlin.
I checked the servers, and indeed they'd functioned all weekend, losing connection to the cameras the moment the power to the site dropped, so I checked the core switch, which had also stayed up, but had lost connection to all the PoE switches the moment the power cut.
It seems the installer had only connected equipment in the security suite to the UPS, so all the edge switches dropped with the power, and all the cameras dropped, as they were drawing power from the switches.
I pointed to a paragraph in my response to the tender, were I emphasised the importance of all switches being connected to the UPS, grabbed my coat and hopped on the next flight home...