In the numerical examples used by Marx to study the reproduction of social capital, the economy reaches a steady growth path in only two periods. This surprising outcome has been interpreted by Marx’s readers as a property of a model which excludes any sort of economic crisis. This paper shows that this is not the case. Depending on the proportion between sectors and on the accumulation rate in the sector producing the means of production, two kinds of crises can occur. Following a suggestion of Marx, we emphasize the analysis of the physical conditions of reproduction and, on such a basis, we determine the critical proportions beyond which a crisis occurs. If, as in Marx, commodities are exchanged at their labor value, the critical proportions will be changed.