When I worked in Texas we used to do bi-annual real production failovers to our backup DC, run for a couple of days and then fail everything back. It was how we knew we prepared for hurricane season. Interesting how many orgs don't consider failback as part of the exercise. One of my peers (at another org) said management considered the risk too high for "just a test". I'm thinking it's actually quite rare to find companies doing real failover/failback testing in their production environments.