[BlackFly]

From the Semver perspective, change to an undocumented (and therefore not public) feature is not a major change. The point is that their safety documentation doesn't change from one system to the next.

Anyone who was "doing it by the book" was not pulling up on the stick.

Now, maybe Boeing was suggesting via side channels that there were alternate ways to solve certain problems and those side channels should qualify as public documentation... but it may have been intuition earned through experience overriding standard procedure.

[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.

[rocqua]

This is a classic case of 'work to spec' vs 'work to practice'. Yes, people should go of the official documentation rather than what works in practice, but it is obtuse to presume everyone will do it in the same way.

Normally, I'd say that those who ignored the spec deserve less consideration. However, when that involves giving less consideration to 100+ passengers as well, that changes.