No line of production code shall be created, modified or deleted in the absence of a failing test case, for which the change is required to make the test case pass.
Various people add extra conditionals to the dictum (my personal favourite is "…to make the test case pass with the minimal, most domain-consistent code changes currently practicable") but, if your shop is (striving towards being) agile, it's very hard to imagine that you're not honouring that basic dictum. Usually in the breach at first, but everybody starts in perfect ignorance, yes?
I (very) recently was working on a bit of code that, for nearly its entire existence over several major iterations, had 100% test coverage (technically, C0 or line coverage) and no known defects in implemented code. It then underwent a short (less than one man-week) burst of "rush" coding aimed at demonstrating a new feature, without the supporting tests having been done beforehand. It then was to be used as the basis for implementing a related set of new features, that would affect and be affected by the state of several software components.
That induced some serious second-guessing. Do we continue mad-hatter hacking, trusting experience and heroic effort to somehow save the day? Do we go back and backfill test coverage, to prove that we understand exactly what we're dealing with and that it works as intended before jumping off into the new features (with or without proper specs/tests up front)? Or do we try to take a middle route, marking the missing test coverage as tech debt that will have to be paid off sometime in The Glorious Future To Come™? The most masochistic of cultists (or perhaps the most serenely confident of infinite schedule and other resources) would pick the first; the "agile" cargo-cultist with an irate manager breathing fire down his neck the second; but the third is the only pragmatic hope for a way forward… as long as development has and trusts in assurances that the debt will be paid in full immediately after the current project-delivery arc (at which time increased revenue should be coming in to pay for the developer time).
The moral of the story is well-known, and has been summed up as "Murphy " (of Murphy's Law fame) "always gets his cut", "payback's a b*tch" and other, more "colourful" metaphors. I prefer the dictum that started this post, perhaps alternately summed up as
Don't point firearms at bits of anatomy that you (or their owner) would mind losing. And, especially, never, ever do it more than once".
Because while even the most dilettante of managers is likely to have heard Fred Brooks' famous "adding people to a late project only makes it later" or his rephrasing as "nine women cannot have a baby in one month", too few know of Steve McConnell's observation (from 1995!) that aggressive schedules are at least equally delay-inducing. If your technical people say that, with the scope currently defined, that something will take N man-weeks, pushing for N/4 is an absolutely fantastic way to turn delivery in N*4 into a major challenge.
Remember, "close" only counts in horseshoes and hand grenades and, even then, only if it's close enough to have the intended effect. Software, like the computers it runs on, is a binary affair; either it works or it doesn't.