George traveled to Six Flags Great Adventure, in Jackson NJ to ride the KingdaKa, the tallest roller coaster in the US, and one that employed every safety measure known to modern engineering. However, while riding the roller coaster, he somehow slipped out of the car, fell 100', and was injured, resulting in $50,000 of damages. The defense presented by Six Flags proved that every possible safety measure known to man had been employed. The juryawarded George $50,000. This is an example of what legal doctrine?