Is it possible to show that a moral claim is mistaken without taking a moral stand with regard to it? A striking number of contemporary metaethicists suppose that it is. In this paper, I argue against a prominent line of support for this supposition. My goal is to cast suspicion on a general tendency to think that the epistemic standing of moral claims is something that can be assessed from outside the practices of making and critically evaluating moral judgements. I do this by focusing on a widely accepted criterion of competence with regard to the use of moral concepts, the moral supervenience criterion (MSC). This criterion holds that someone who judges two acts or events to be morally different without thinking that he has to identify some non-particular non-moral difference between them simply doesn't understand what it is to make a moral judgement. I focus on a paradigmatic example of the sort of mistake in moral judgement that is supposed to support the MSC and argue that it provides no support whatsoever. I then offer my own alternative explanation of this sort of mistake in moral judgement. I conclude with a discussion of why advocates of the MSC are inclined to suppose that it is possible to assess the epistemic standing of a moral claim without oneself taking a moral stand with regard to it.