We seem to have replaced cooling and power and a grumpy sysadmin with storage and architects and unhappy developers.
That would have been a major problem if we'd had a nighttime power outage.
After that we ran regular switchover testing :)
The other time we ran into trouble was after someone drove a car into the local power substation. Our systems all ran fine for the immediate outage, but the power company's short term fix was to re-route power, which caused our voltage to be low enough for our UPS batteries to slowly drain without tripping over to the generator.
That was a week or two of manually pumping diesel into the generator tank so we could keep the UPS batteries topped up.