I used to work at a regional bank. In 2000 or so, our group got a call from the corp. office wanting some assistance reloading an OS/2 2.1 server. Seems it had been sitting in a corner somewhere doing currency conversions for some department and everyone sort of forgot about it until the hard drive finally died.
OS/2 Warp (3.0) came out in early '95, Warp 4.0 came out in '98 or so. We're guessing the system was running for 3+ years with no intervention at all. Scary stuff.