Reminds me of a tale on usenet years ago about a new hospital being build - the doctors had the say in what went first onto the battery system in case of power outages. Idea of course being vital equipment was on this, and everything else would be handled when the big generator powered up.
IT got power for the systems and yes, even AC was powered.
Testing the genset worked every time, till they got a real power outage.
Vital systems went onto UPS, and there was the sound of the genset firing into life.. for a few seconds, then it coughed and died.
Turns out what wasn't deemed "vital" was the lift pump for the main diesel tank - it was on the backup power system, but not the batteries.
So it fired up using the diesel in the pipes, and the pump would just come on and start to pump diesel up to the genny.. and it wouldn't get there before the poor thing died from fuel starvation.
IIRC the fix in the end was a new fuel pipe, which happened to be something like 10x the diameter needed.. and hold enough fuel for the genny to start and new fuel to be pumped up, as they weren't allowed a header tank - but a bigger fuel pipe wasn't a tank!