I have blogged before on the very deceptive claims that the immediate post-WWII period causes problems for Keynesianism. It clearly doesn't pose a problem for Keynesian theory in the slightest (though some still claim it does). The one leg they have to stand on is if maybe Keynesians at the time were worried about it.
Paul Samuelson is usually their go-to guy, and his chapter in the Harris volume is their go-to source.
My response is usually to point out that not only was Samuelson extremely atypical in his position in this piece - he was aware of it and repeatedly made the point in the Harris volume that he disagreed with the consensus Keynesian position (Brookings, Klein, Beveridge, Keynes himself, Kaldor if I recall, the CFD, Hansen, etc.).
LK has a new post up with extensive quotes from the chapter and makes an important point that I usually don't make: Samuelson actually wasn't as pessimistic as he's often portrayed. He expected some trouble but wasn't predicting a return to the depression or anything like that.