I worked for a company that thought it had a really top level power backup system. The server room had 15 minutes of UPS, all PCs had at least 5 mins. Out the back was a big generator with 24 hours worth of diesel. It was all controlled by a fancy control system that fed light displays in every room indicating if we were on mains power, UPS power or generator power.

One day during a power cut the generator kicked in as it was supposed to, but a minute later it spluttered to a halt. After much fiddling around by maintenance staff they gave up and we were all told to go home.

The postmortem revealed that the diesel had become contaminated with all sorts of gunk - water, grit/sand and lumps of unidentifiable "stuff". The best laid plans and all that.

Edit: And cigarette butts as well.