During security assessments, I always make sure they’re performing security testing as part of their development process.
No need to go into details as to what happened here; it’s well-researched in the linked article. However, this is exactly the scenario that development security testing is meant to avoid. A seemingly innocent patch disables or circumvents an important security feature. The results are predictable.
It could be worse, though. Here’s the worst case: the problem isn’t detected. Because the security was included in the original version, and because nobody checked, it is assumed that the security is in place, and successive updates are made, with the security feature in question not working, but everyone assuming it does. And successive patches are built upon the circumvented security. By the time the bug is discovered, fixing it is a gargantuan task.
So, it’s not that bad. It’s still a major breach, though. So if you ever wonder if that testing is really necessary during development, you can point to this incident and confidently say “Yes.”