Not a truly autonomous vehicle example but this is a case where most likely the car reacted before the driver was even aware of a problem: https://www.youtube.com/watch?v=APnN2mClkmk
I agree with the sentiment though. This has been a major selling point for this technology, but it has not been sufficiently demonstrated yet.
From the video, it looks like the car noticed the crash at the same time a human would, but the car decelerated immediately, whereas the human might have slower reaction time.
(It certainly didn't predict the crash "seconds before", as the car accelerated into the imminent colission, until <1sec before collision.
I agree with the sentiment though. This has been a major selling point for this technology, but it has not been sufficiently demonstrated yet.