Many public pension insurance schemes today use the pay‐as‐you‐go financing mechanism. This mechanism is vulnerable to an ageing population, which puts pressure on the intergenerational contract implicit in these schemes and raises the question as to how they might be eroded. This is not a new problem, and to put it into historical perspective, this article studies the intergenerational contract that formed the core of the Prussian miners’ invalidity insurance in the nineteenth and the early twentieth century (1861–1920). With the so‐called Knappschaften, miners relied on what was probably the most comprehensive and advanced occupational pension system existing in Germany around the time when Bismarckian social insurance was established. Financed via the pay‐as‐you‐go mechanism, the miners’ pension funds faced stress from their ageing memberships early on, and this potentially undermined their ability to maintain intergenerationally fair pensions. In order to examine whether or not the intergenerational contract among German miners showed signs of erosion, we look at the Knappschaftens’ profitability, as measured by actual and promised internal rates of return. This article shows that the intergenerational contract indeed weakened over time unless miners’ funds were large and continued to grow, and that a pension reform in 1906 served to stabilize generosity.