A CIO friend of mine recently went through an experience that we should all learn from. There was a power glitch at his data center and, as it was designed to do, the UPS took over. Unfortunately, one of the batteries was bad and caught fire (yes, they can do that). Fortunately, the fire put itself out and nothing else was damaged. One week and four hundred new batteries later, the UPS is back online and everything is back to normal.
The batteries were on a scheduled maintenance cycle, but this one failed before its scheduled time came up--it happens. The moral of the story: if you don't have SNMP-enabled battery monitors in your data center yet, its probably worth the investment. Don't count on scheduled maintenance to keep you up and running.
Another important lesson learned was that dealing with an abnormal electrical event required on-site electricians to rewire some things on-the-fly. Make sure you've got an electrician on call who's familiar with your data center before the crisis. It's no fun to find out that things aren't wired the way you thought they were. Unfortunately, a crisis is usually when these kinds of problems will come to light. Be prepared to handle them whenever they come up.