I’m pinched for time, so I’ll unashamedly pinch this from Bishop Hill (who I don’t think will mind):
We shall argue that consensus among a reference group of experts thus concerned is relevant only if agreement is not sought. If a consensus arises unsought in the search for truth and the avoidance of error, such consensus provides grounds which, though they may be overridden, suffice for concluding that conformity is reasonable and dissent is not. If, however, consensus is aimed at by the members of the reference group and arrived at by intent, it becomes conspiratorial and irrelevant to our intellectual concern.
Both the paper and the blog post are must-reads.