Consider the statement: "a government's just powers derive from the consent of the governed".
Is this statement true? It seems not, on first brush. After all, it is the very nature of government to assert coercive authority. And yet it seems true at least in the sense that a government cannot function unless enough people go along: it seems true that a government's powers derive from the consent of enough of the governed to make it stick.
But I was a little sneaky in that last paragraph, because I ignored the word "just". And it is here that the formula really starts to run into trouble. Because when it is just for me to obey my government, my consent doesn't play any role whatsoever in that justice. My obedience expresses my own justness or lack thereof: a just command from my government carries normative force indepenent of my willingness to obey it. When the government is telling me what I want to hear no doubt I consent, but the government power which requires justification is precisely its authority, its power to compel: its power to make my act morally normative independent of whether or not I want to do it. So the idea that the just powers of a government derive from the consent of the governed seems straightforwardly false.
On the other hand, it is straightforwardly true that a wicked government has no power, as a normative matter, to compel obedience to a wicked command. That such a government may kill you if you disobey doesn't confer even a tiny bit of normative force to its command. It isn't possible - it isn't even conceivable - to have a moral obligation to do wrong.
There are in between cases which remain problemmatic. A government may command something that is not a wicked thing for me to do, yet it may still be an unjust command. It isn't wicked for me to sign over all of my property to my rich neighbor, but it would be tyrannical for the government to insist that I do so. The reason why is not the libertarian notion that I am the demigod of my personal property, but rather the notion of subsidiarity. My property is a local concern of mine as its legitimate owner. Subsidiarity as an imperative insists on local sovereignity where it is possible: that affairs which are manageable through local control should remain local affairs. Tyranny isn't a violation of the personal autonomy of the superman: it is a violation of subsidiarity.
And therein lies the heresy. "Consent of the governed" rings true because of its proximity to a legitimate imperative. But as with all heresies, it mixes the truth with a lie.