This was a year-long several-hundred rack problem that culminated in a hellish weekend of running cables and powering things up. It was "all hands" in IT and didn't matter that I worked in a capacity that kept me far away from hardware most of the time -- I had past expertise in that area, I was called upon.
Everything went relatively smoothly until about three months later when I received a phone call from our department VP. Apparently a vendor that we billed for Cost of Access[0] had not paid us since we moved DCs; they hadn't received any bills and were happy not informing us of the problem. A long investigation resulted in the discovery that the bills were submitted electronically, by modem (this was 2008, not 1998). The modem phone line was moved and located at the new datacenter, hooked up to a USRobotics 56K modem, which faithfully dialed a number, connected at 1200 baud (?!) and submitted the bill. And we're talking a bill that's big enough to be noticed on quarterly reports.
The offending computer had been brought to my office building in the suite downstairs, and my VP said "You've got some experience with old hardware, can you just take a stab at it?" It wouldn't boot; the CMOS battery was bad. I noticed three numeric values printed on a label affixed to the case and realized, immediately, that these were IDE (ATA) drive geometry settings. So the CMOS battery failed a long time ago, and the engineer solved it by printing the numbers on the front of the case.
And what a PC it was ... an IBM NetVista circa somewhere near the year 2000. Running PC-DOS. It sat in a rack on a shelf surrounded by servers, completely ignored since it was hooked up to UPS, not accessible to the corporate network, and was never patched or rebooted[1]. It was an accidental somewhere-near-3/4-million-dollar-desktop-server.
[0] Cost of access for the kind of telecom that we were was, I think the largest non-staff expense the company had, but it also worked the other way -- we billed people CoA for our network.
[1] The device could not be accessed on our network except from one other host, via one port; it was as air-gapped as it could be and other than the few minutes spent "sending the bill" each month, it didn't carry any data of value.