All of these parts make for an increasingly complex system with a large number of failure points.
Our DC was a very living entity -- servers were being changed out/rack configuration altered very regularly. Large operations were carefully planned. You wouldn't overlook the power requirements of a few racks being added -- there'd -- literally[0] -- be no place to plug them in without an electrician being brought in. However, in a 3-month period every once in a while, two racks would have old devices replaced either due to failure or refresh, one at a time.
Since they weren't plugged directly into rack batteries (we had two battery rooms providing DC-wide battery backup), the overload wouldn't trip. Since we were still below the capacity of the circuit, the breaker(s) wouldn't trip. And maybe we're still under capacity for our backup system, but a few of the batteries are under-performing.
I think the lesson we learned when this happened was: you need to "actually test" the thing. My understanding is that our tests were of the individual components in isolation. We'd load test the batteries and the generator and then the relays between. At the end of the day, though, if you don't cut the power and see what happens you don't truly know. And my understanding is that having that final step in place resulted in a large number of additional tests being devised "of the individual components" that ensured they never had an outage like that, again.
[0] Guessing it's common practice to make "finding a f!cking power outlet" nearly impossible in DC. Every rack had exactly the number of leads it needed for the hardware plugged into a completely full receptacle. They rolled around a cart with a monitor, printer, label printer, keyboard, mouse and a huge UPS on it so staff could do daily maintenance work.