I mentioned some time ago the trucks filled with computers and backup generators parked outside Building 26 on the evening of December 31, 1999, standing by to assist with operations should a worldwide disaster strike during the change to the year 2000.
(Spoiler: Nothing major happened.)
A now-retired colleague of mine¹ told me that those trucks were originally there for a different reason.
Those trucks with generators had been parked outside Building 26 for months before the end of 1999. They were originally there for reliability testing.
The reliability goals for Windows 2000 included continuous uptime that exceeded the time between power failures in the Redmond area. Although the Pacific Northwest is rich in hydroelectric power,² we also suffer from a lot of power outages during the windy autumn and winter months. That's what happens you have a lot of tall trees, I guess.
The original plan was for the trucks to be returned when the testing was completed in December, but upper management decided to extend the lease through to New Years, just in case.
A lot of critical infrastructure needed to be updated with a stable operating system well before the end of 1999. The software that runs banks and hospitals are closely-regulated, and a high degree of continuous reliability is one of the criteria. Customers in those industries (as is typical of most industries) have a lot of third-party software that needs to be tested. This software is highly specialized and won't exist in Microsoft's application compatibility library.
Enter the trucks with computers and backup generators. These systems were set up to replicate the necessary computing environments and set to task running tests, including so-called "long-haul" testing, which keeps a system running for extended periods to help flush out issues like a tiny memory leak that eventually grows to the point where the server becomes unresponsive.
My colleague notes, "There were a lot of extra hours put in to make sure the industry was supported for an event that would happen only once."
¹ In the years leading up to his retirement, my colleague was, among other things, responsible for maintaining Notepad. He wasn't the original author (the identity of whom has been lost to the mists of time), but he was the one in charge of keeping it running.
² Although coal also plays a major role.