I do remember a story that did the rounds many moons ago. Apparently a computer facility somewhere in the US decided to test their backup power systems by physically shutting down the mains power. Everything started OK; power went off, UPS takes up the load, then the diesal generators kick in and take over from the UPS.
Everything works absolutely swimmingly. For about 30 sceonds, after which point the generators stutter and die, followed shortly afterwards by the UPS which did not have the capacity to keep the facility going for long (they wrere only sized to handle the load until the generators spin up to full capacity). Queue "oh sh*t" moment.
After a long-ish investigation the source of the problem was identified. Apparently the fuel tanks for the diesal generators had to be located some distance away for fire safety reasons. Because of the distance a gravity feed to the generators was out of the question, so they used fuel pumps instead. Which (you may have guessed it) where powered directly from the mains with no feed-in from the backup power systems. Hence the generators worked for until they have sucked the fuel lines dry, then shut down.
Queue secid "oh sh*t" moment plus much forhead slapping.