Real engineers are like true Scotsmen, because you can never have enough process.

In this case, of course tests really do prove the absence of bugs rather than just (not) proving their presence.

I really don't like the whole "unit tests lets you not worry about breaking things" view. That's not confidence, it's overconfidence. And it doesn't even mostly work for things where "correct" is more fuzzy than "eventually produces this exact output".

It is confidence. It is the right tool for the job.

What would you propose as an alternative?

I'd be surprised if they didn't make postmortem changes to prevent such incidents in the future. Your posts in this thread imply that no mistake should ever happen, but let's be real - mistakes will always happen, the key is to do proper postmortem analysis and learn from them (and obviously prevent recurrences).

Some mistakes, yes. But this was a case of such a basic and clear cut use case that should have been tested.

My understanding is that companies such as this rely purely on manual testing. Thus it is easy for things to slip through the cracks. Usually they never get hit by penalties or get caught so the pressure to do better is not there.