Unless you push out a bug that is triggered by some combination of events so distant from the release that you could never have caught it in testing. Perhaps a

Unless you push out a bug that is triggered by some combination of events so distant from the release that you could never have caught it in testing. Perhaps a bizarre interaction between multiple unrelated systems from different vendors that all work tested in isolation and only fall over when a series of 1 bits in a certain pattern in the real time clock chip creates a blip on a bus that gets corrected in a way that was interpreted differently by two different teams and the fallback system tries to take over but the primary system comes back online at just the wrong time and trips a bus contention bug that was never seen because testing never hit this perfect storm of race conditions and, well, the front falls off. The point of me making up a stupidly improbable story is that stupidly improbable events keep happening despite all the clever people in the world. And of course the people on hacker news who know even better.