Ben Myers is stirring up some fun again - here.
I'm with Steve that the doctrine of penal substitution (tightly defined as post-first millenium) is one of the worst. But actually, what I believe is the worst is to do with the Eucharist - the shift in meaning of 'The Body of Christ' in the Western church in the first two hundred years of the second millenium. I think that is responsible for almost all the disasters that have happened to the faith since then - including penal substitution and biblical inerrancy (which is what I voted for). But I'll say much more about that in week 3 of my learning church sequence (Cavanaugh talks about it, but our present understanding stems primarily from the work of de Lubac).