@Avionker
To my mind a design failure is something which makes a component or system, when installed correctly, unfit for it's intended purpose. What is your definition exactly?
When the failure of a component or system leads to a hazardous condition.
Things fail. System designs need to take this into account. In the case of United 811, the system had a failure mode where a short circuit resulted in the uncommanded unlatching/opening of a cargo door in flight rather than tripping a c/b or otherwise leaving the system in an inoperative but otherwise safe state.