Not a specific answer for a Toyota Prius, but:
This summer, we had an incident here in Germany, where the motor stopped after throwing the key out of the window.
There's an online police report, so this is not a hoax.
Here is my short translation:
A 33 year old mother was driving her VW Passat on the highway (German "Autobahn") A1 near Lübeck with her 1.5 year old daughter on the right rear seat. As the daughter was whining due to tiredness, the mother gave her the key ring with the ignition key to play with. As the daughter didn't calm down, the mother opened the window a little, hoping the fresh air would help.
The daughter threw the key out of the open window, causing the motor to stop. The car was still steerable and the mother managed to stop on the emergency lane.
The mother called the police, who searched for the key ring and finally found it. All keys including the ignition key were damaged, but the electronics survived, so the mother was able to drive home.
So, this was a Volkswagen, not a Toyota, but it shows that bad thinks can happen.
Of course, the behavior of the car depends on what the engineers implemented and may vary from brand to brand, or from car to car.
In my personal mind, the motor should not stop while driving, but on the next halt.