The last lesson held a deep stance with a controller. This one asks what happens when the controller stops. For every machine classical safety law was written around, the answer is that it becomes safe: a protective stop removes power, and the machine coasts or brakes to a standstill that then persists with no energy and no computation behind it. That is a Stop Category 0, and it is why a fixed arm, a collaborative robot and an AGV can all be certified today. A balancing biped inverts the assumption. Remove power from a walking humanoid and it falls, so the mechanism the law relies on to produce safety is itself a hazard. A 2026 study from Siemens Foundational Technologies names this the fail-passive gap, and instead of proposing new hardware it measures around the gap: build the entire external chain from already certified parts, account for every term in it, and whatever is left unaccounted for is exactly the part that cannot yet be certified. What is left is the robot's own reaction. You are going to price it. ISO 13855 fixes how far a light curtain must stand from a hazard as S = K*T + C, where T is the whole time from the beam breaking to the machine being safe. For a biped that T has to carry the time the balancing policy needs to reach a standstill, which is the one term with no certified figure behind it, and which the study measured rather than specified. Compute the fence with that term and without it.