If you forget to set the parking brake, you were negligent
If you set the brake but it didn't work, the car malfunctioned
From the link you sent,
> One of the questions herein was whether there was a defect in the automobile mechanism for locking the transmission gears when the automobile was in a parked position.
> (...)
> The parking lot sloped in that area, and he put the gearshift lever in "park lock," and went into the building.
> (...)
> In response to a hypothetical question, based upon assumed facts justified by and embodied in the evidence, Mr. Nass testified in substance that it was his opinion that the automobile rolled down the slope of the parking lot because the transmission internally was not in park-lock; that it was not in park-lock because the engine was "moving around"; and that the engine and shift console were not in proper synchronization because the motor mounts were not restraining the engine and were not holding the engine in position.
That case had the owner performing an action they were led to believe would have prevented the car from rolling but potential defects prevented it from doing so.
