There is something that's called fail safe(ly). In case of any error inside the car, it should sowly decelerate and pull over. Sure, it will cause a lot of traffic problems if say 10% of all cars did that at the same time but the damage would not be as severe as a ghost driver entering the freeway with 140mph.

There are a few instances where some bad tesla batteries (the standard 12 volt batteries ironically) failed, and the cars handled it perfectly. It slowed down so that the driver could do safely pull over. Sure it did not happen to all cars at once, and autonomous cars migh not be able to do that by themselves but we have a log way to go to reach 100% autonomous driving (i.e. Without a steering wheel and a car that drives everywhere humans drive and not only on San Francisco's perfect sunny roads where it's been thoroughly tested on).