I have a good guess on how this would actually happen:

PM: We need this

Specialist: makes this (doesn’t check results)

QC: Looks good (but doesn’t actually check)

Some updates later may further break the functionality. And as long as numbers aren’t blatantly wrong (think 0s everywhere, for example) and nobody checks thoroughly enough, the issue will remain.

I have unfortunate experience of being a part of such a story, haha. There are ways to counter it. Mainly, their project documentation either wasn’t up to par or wasn’t used as a reference during creation and tests. Either way, it’s negligence.

My tipping point was YouTube serving the same one ad months on end to me.

I was fine with ads a decade ago. I tolerated them increasing the amount five-fold over the years. But that was borderline torture, and getting an adblock was the only working solution.