Long ago I was working for a programmer writing an accounting system for a client (I wrote the user docs). The machine we were replacing was an old NCR plug-board system. Programmed by wires plugged into holes on a board. One person left at the client knew how the machine worked. No one knew how the programming worked. If the wires fell out of the holes it could not be put back together again. At one time someone knew that, but the knowledge was lost. This is the system we are building again today.