A Valve engineer fixed 3D lighting so hard he had to tell all the graphics card manufacturers their math was wrong, and the reaction was: 'I hate you'

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.