The data center where I work self-tests this stuff unintentionally a couple of times a year. The typical case: UPS maintenance, room is put on bypass, load drops when switching back.
This is arguably better than not having any tests.
This is why I reboot my server from time to time after having applied patches or made more significant changes, despite the fact that "it should not change anything". This is a good moment to realize that it did change something and you have the opportunity to fix the issue while it I sfresh in your mind, and possibly with more time.
