Background Information It was then a principle of defensive design anticipating the mistakes the end-user is likely to make. For example, no competent designer would make a symmetrical two-pin plug and then label it " THIS WAY UP if it matters which way it is plugged in, the design should be asymmetrical so it can' t be plugged in wrongBackground Information It was then a principle of defensive design: anticipating the mistakes the end-user is likely to make. For example, no competent designer would make a symmetrical two-pin plug and then label it "THIS WAY UP"; if it matters which way it is plugged in, the design should be asymmetrical so it can't be plugged in wrong