Theorists of law and of social policy distinguish between the efficacy and the impact of laws and legislation. Efficacy concerns the achievement of the effects intended by law-makers. Impact refers to the actual consequences. At times, the two diverge. Sometimes, it is only after a lengthy period that the impact is appreciated. The impact sometimes requires intervention by legislators or judges acting purposively in order to correct unforeseen or unfortunate consequences. The extent of judicial intervention is not always clear from the surviving records. This article argues that an awareness of the possibility of such occurrence is essential if legal history is to present the whole truth concerning legal developments.