Lady Harry was involved with a major IT operation a few years back for a very very large organisation
Every time they tried to move things they discovered another set of old software cheerfully doing it's job and totally undocumented by the IT guys. I think the record was some 1962 stuff. Some of it had a UNIX/Windows front end bolted on but that was purely for look and feel
It was so much part of the users day to day they'd effective forgotten about it - it was like the light switches - always had been there, always worked, never failed.
Oh, and of course it was lightning fast on modern machines..............