A posteriori vs A priori
In Ontology is Overrated: Categories, Links, and Tags, Clay Shirky clearly explains the issue of a posteriori knowledge versus a priori knowledge -- the essential theme of austrian economics and libertarian politics. However, in the end, he not only fails to conceptualize the issue, he also steps into the trap laid by relativists: he confuses reality with knowledge about reality (a clear case of insanity, as defined by general semantics), and implies that we create meaning rather than discover it.
The nuance is tiny, but as Aristotle said, the least deviation from truth will be multiplied later. Human creation of meaning entails that meaning is an arbitrary product of human will, and that any disputes arising from divergent opinions cannot be solved by reason but only by brute force. Discovery of meaning entails that meaning is objective, and that disputes can be resolved by reason and other peaceful means. Thus, what appears as a mere detail in something as fundamental as metaphysics and epistemology has dramatic consequences when you later study ethics and politics!
Of course, Clay doesn't explore the political consequences of this tiny detail in his essay. His mistake is benign, and I'm sure he himself would be prone to dampen the mistake rather than amplify it, if he were to be taken in a more political discussion. Still, by admitting such wrong premises, Clay abandons all possibility of arguing back against those people who preach Evil. If he's not doing a disservice to himself, he is doing one to his readers. He is both a victim and a disseminator of one of the fundamental memes by which Evil disarms good people.