Essentially your scenario includes many factors that need to happen consecutively. The most catastrophic failure is the snapping of a brake shaft on a geared machine which I have never heard or seen happen. If this happened then the stopping of the car in the up direction is unlikely as safety gear is generally only operable in the down direction. Having said that newer MRL safeties are now bi directional. This is why you engineer to specs(hopefully) and incredibly rare if at all. Un intentional movement can happen. We had a car take off from the top floor into the top of the shaft(before my time) and I think an issue was created when the main contactor in the loop circuit flex earthed down enough to create a dynamic braking situation that had enough of a potential difference to create a current to move the car. I don't know enough of the situation to comment further but this is the only thing I ever heard of that was similar to what your asking.
