In the spirit of the question What is your oldest hardware that still works?, I'd like to ask:

What is the oldest hardware you know that is still in production use? And what challenges did you (or someone else) face in keeping it running (scarce documentation, no support, no spare parts available...)?

Most organizations will retire / upgrade software and hardware after 5-10 years, but sometimes old software is kept running on old boxes, because it "just works".

I once worked at a client site that was running a critical piece of (in-house developed) business software on a single server running HP-UX. The server was old (ca. 12-13 years), but fortunately still running without problems; however, getting spares would have been very difficult, and since software installation was undocumented, any significant system changes or even new hardware might have caused significant downtime and data loss. We eventually managed to replace it, but this is not always possible.

I also read that many organizations still run decade-old mainframe hardware, particularly for highly customized systems controlling industrial machines or power plants.

Which old hardware have you encountered? How did you manage these challenges?

Related question:

http://serverfault.com/questions/82467/should-old-servers-be-retired