No one is behind the wheel. Right now there is no mechanism for a third party such as law enforcement to reliably stop or redirect the vehicle.* Yes, it is possible that an issue could occur and the vehicle will keep driving.*
* you could say the same thing about human motor vehicle operators
Driving my kid to/from school in San Francisco, I regularly see completely empty Waymo cars driving around... pretty much every day. There's 1-3 that I see parked on a particular wide and relatively empty side street and I suspect they use that street to wait until they have a destination.
There are multiple failsafes to prevent a “keeps on driving from a software bug”.
There are overseers who are remotely monitoring vehicle telemetry. If there is an accident, the company will know before anyone else and will respond accordingly.
In the extremely unlikely event this happens and the AV doesn’t pull over the footage is saved for a time so the other driver/pedestrian/etc can simply contact the company/police for insurance claims.
* you could say the same thing about human motor vehicle operators