The "Prime Directive" of Agile Development

Keeping the system executing at all times is the prime directive. You never do anything to the system that will break it for more than a few seconds. If you have a huge architectural change to make, you find a way to make that change in tiny little steps that keep the system executing. You find a way to keep all the tests running. You never do anything that breaks the system for more than a few seconds or minutes.

Source: The Prime Directive Of Agile Development

Easier said than done, but a worthy goal nonetheless.