That was a strange one. The car was a loaner, and had the wrong carpet in it, and the carpet got stuck. “One of the family members called police about a minute before the crash to report the vehicle had no brakes and the accelerator was stuck.”
There is no rational failure mode that would stick the accelerator AND disable the brakes. The brakes are not fly-by-wire, if you stomp them it will apply the brakes. You can also lock the emergency brake if necessary.
Also, these push-button cars still let you put the car in neutral, although with a stuck accelerator that could overrev the engine.
Turning off a car while driving can be a problem, since it could lock the steering wheel. Toyota said “put it in accessory power”, but with these pushbutton cars, you have to power them off and then power them into accessory, which requires that you take your foot off the brake or it just turns the car back on.
In the Prius, the gas pedal is not only “fly-by-wire”, it doesn’t even directly control gas to the engine, because the car will only turn the gas engine on if it neeeds additional acceleration.
Loose carpet accordions up under the brake pedal but over the accelerator is one mode that quickly comes to mind.