Account takeover after a key change
Testing has always needed two things: a way to search for problems, and someone to decide what "correct" actually means for this code. The searching part has gotten faster and cheaper — first with automated tests, now with AI. The deciding part hasn't. It has always come from whoever understood the intent behind the code, and as more code ships faster than any one person can hold that intent in their head, that decision is the part nobody has actually solved for.