There are a couple of negligence theories in product liability/faulty product cases: Proof that the product was defective as manufactured when it left the factory (this is done by tracing the product condition back to the time it was manufactured, with no changes or significant maintenance, which can be the proof of a defect as manufactured). Or it can be shown that the manufacturer negligently failed to warn you or general consumers about a known product defect, and that the manufacturer had the ability to get the word out by letters, by Internet or other means.