knb:

A good rule of thumb is that if two absolute ideologues like Paul Krugman and Milton Friedman agree about an issue, there must be pretty damn good evidence.

I happen to agree with them about the issue of rent control, but taken as a general rule, this is absolutely fallacious. Within any belief system, leaders of different factions will often hate each other ferociously over their differences, even though from an outside perspective, they may well look almost indistinguishable. Just look at various religious and ideological disputes over incomprehensibly obscure points of doctrine.

(And yes, I do think that modern economics has some fundamentally unsound beliefs that are a matter of virtual consensus nowadays. See e.g. this discussion for some examples.)