Although the Hymotion system cuts power to the Prius when it's plugged in, that's not foolproof either.
It requires that AC power connects all the way through. If the other end of the extension cord isn't plugged in, nothing stops you from driving off with the cord.
Outdoor outlets have a GFI on them. If that pops, BOTH ends of the cord can be plugged-in and you still don't have power which activates the "No, you can't drive" feature.
My wife somehow managed to drive off once with the cord plugged into the Hymotion charging port. I wasn't home at the time. When I got home, the cord was out on the driveway, pulled hard on the electric outlet. The female end of the cord had a little damage on the ground pin. Thankfully, there was no damage to the Hymotion port on the car.
As far as I could tell, the cord was only pushed half-way on to the car, and she just forgot about unplugging it. (Busy with the baby, etc.) Since it was only half-plugged-in, the car's safety feature did not kick in. On the other hand, at least the cord came undone with little damage.
Long-story short; even fail-safes fail, and low-tech beats high-tech every time.
I like low-tech.
