Power Supply Work
Power supply work is the main impediment to any record-breaking 100% uptime. I've never worked anywhere that hasn't had to have down time due to some kind of power supply work. Either locally within the office, or the building, or the entire street. And I'm talking about planned, prolonged interruption, something that can't be safely handled by a UPS.
To find somewhere that's managed to avoid both power supply work and power failure for 18 years is remarkable. It makes it a remarkably durable server, but saying it's been on constantly all that time seems a bit unlikely. If it was part of such a critical service that benefited from such a guaranteed and protected power supply, then surely they wouldn't be relying on running it on such ancient hardware?