I have always been baffled by clients that have expensive systems installed and the moment something doesn't work (years later) claim one of two things:
1 - it should still be warranted because in the many years they have had the system they have only ever used this item a couple of times so it should not have failed.
(maybe if they used it more often within the warranty period it would have failed then and been replaced for free)
2 - it never worked right. (so what they are saying is that if the radio in their car did not work right, they would wait a few years before bothering to have it looked at, and be mad at the dealer for selling them a car with a broken radio)

It is only going to get worse as our clients get older and older.
They will start to forget how to operate certain things they don't use daily.
You can go over how a system works and tell them everything they need to know, but they won't actually be listening.
You can write everything down in a nice document, but they won't read it.
They will only call you and once you have "fixed it" and shown them again how to use something, they will claim you never told them that before.