Another big gotcha that hits large public organizations is that the software must be ADA compliant. This is extremely difficult and expensive to meet. Most small vendors do not meet the minimum requirements or even consider doing so.
That was an uncomfortable day. ;)
Once you cross a time zone, your records are much harder to access.
There were many potential patient safety issues due to mishandling of times during that hour and the suggestion to just use the backup worked because it happens at 1 AM when little is going on other than emergencies which simply have to to manage with read-only access during that time.
I kinda like the idea of regular scheduled downtimes of stable systems, just for real experience on downtime procedures.