[kelnos]

That ignores the fact that people will rely on undocumented behavior anyway, and a responsible developer should keep that in mind.

With a normal software library, you might make the decision to cause breakage anyway, even if it inconveniences users of your library. Or you might not, because you believe the inconvenience will be too great, and instead just document the behavior and make it a part of the API.

With an airliner control system, you need to be a bit more careful, since a pilot depending on undocumented behavior may do so in a way that could cost lives if that behavior is changed. Is the pilot correct to depend on that behavior? No. But that's irrelevant when lives are at stake.