Story time: A few years ago I was working on a web app for a US intel/LEO agency in northern virginia. The app had started as a demo, then kind of grew. Like a fungus. It was never really designed, much less designed to shut down and restart unexpectedly. There were some other similarly "designed" apps running in the data center.
The data center, being under the flight path for an airport, had a continuity of operations ("coop") plan and hardware. The "UPS" was a big generator with a switch so that it would take over when mains power went down. There was also a system designed to handle hot mirroring of everything and switch all network traffic to the backup center if the main center went down.
A great system which was never tested because what if the test takes the system down for 15 minutes and we thus miss the opportunity to prevent the Next 9/11 and Thousands Die and, worse yet, we have to testify in front of Congress?
So one day the fire marshall came through the building and, as part of his testing, hit the Big Red Switch. The switch designed to detect this and start the generators (and which was reported to cost $15) failed. All the systems went down, hard. The network switch in place to notify the hot backup site and send all the traffic there also failed. And the Vital Systems Protecting Our Nation From the Next 9/11 went down, worldwide.
Don't just have a plan, test it.
p.s. We never were able to determine how much, if any, data was lost....