They've mapped the roads and they don't need to drive into a ditch just because there's a new sign. It probably wouldn't be all that hard to come up with criteria for saying "this new sign is suspicious" and flag it for human review. Also, Waymo cars drive pretty conservatively, and can decide to be even more cautious when something's confusing.

Someone could probably do a DOS attack on the human monitors, though, sort of like what happened with that power outage in San Francisco.