I've had ChatGPT find some pretty esoteric bugs in ways that shocked me.
Like semi-jokingly asking it to "improve" some code thinking it'd come up with some non obvious control flow... then instead having it immediately point out a subtle bug along the lines of "the code sets flag A instead of setting flag B on <insert line>" flag B wasn't even unused, so it's not like a simple unused variable heuristic would have caught that.