I say this a lot when training: "I can never be as creative as our customers."

It's my way of explaining why I always prefer training them in a live environment, rather than one where I control all of the scenarios.

I imagine it is the same way when developing software. You try to account for everything you can think of, and test to make sure everything is working well, but your users will always find ways to break things that you hadn't thought of, and may be difficult to diagnose, let alone find a fix for it.

It's just another way of saying, "No plan survives first contact with the enemy," I suppose.