The coding side of this isn't even the problem - that a broken change like this made it to production is an operational failure.
Why didn't testing catch this? Why was there no canary? How come alarms didn't wake up an on call as soon as the call-volume on the backend dropped? Why was this update rolled out to every customer at once, instead of gradually?
I think verification is still a grey area in the agentic software dev. QA and ppl testing it often resort to coding agents for the QA work, and we all know how good agents are at convincing themselves.
Why pay for and bother with all of that when you can outsource it to your users?
Users who will forget about it, and due to inertia stay on your platform.