> It is realistic to imagine that one printed page stuck on a lamppost could cause the car to reliably crash.

Realistic, yes. But that'd still be a symptom of architectural issues in the software.

Conceptually the priorities of a car are (in order of decreasing importance) not hitting other moving or stationary objects or people, allowing emergency vehicles to pass unhindered, staying on a drivable surface, behaving predictable enough to prevent other road users crashing, following road signs and traffic laws, and making progress towards the destination (you can argue about the order of the last three). Typically you'd want each of these handled by their own subsytem because each is a fairly specialized task. A system that predicts the walking paths of pedestrians won't be good at finding a route to Starbucks.

The "follow road signs and traffic laws" is easily tricked, like in this article or by drawing road lines with salt. But that should never crash the car, because not hitting anything and staying on the road are higher priority. And tricking those systems is much harder