For Mises, only one sin was unforgivable: lack of integrity. Speaking and behaving against one’s better knowledge could not be excused. This is why Mises maintained a lifelong friendship with Gottfried Haberler, whom he believed to be an honest interventionist, but broke with Fritz Machlup when he started qualifying the case for the gold standard. At the same time Mises recommended Leland Yeager for Mont Pèlerin Society stipends even though Yeager was no less skeptical of gold money than Machlup.
J.G. Hulsmann, Mises: the Last Knight of Liberalism http://www.mises.org/books/lastknight.pdf, p. 1000.
