as a developer, you dont ever do a bunch of changes all at once. it makes it impossible to see where a new problem may have come from. you 'lock it down' as fully tested and functional, and then as you build new stuff, you can be mostly confident that it was your new code that broke it. we call them 'sprints' and like 'story archs' and all kinds of abstract BS for non-tech people to understand, but in the end, its really just about being smart about how you fix things, and not letting the business-people or customers drive your release schedule, because they always want everything now, and always will.