How the cure for scurvy was found and lost again.
They had a theory of the disease that made sense, fit the evidence, but was utterly wrong. They had arrived at the idea of an undetectable substance in their food, present in trace quantities, with a direct causative relationship to scurvy, but they thought of it in terms of a poison to avoid. In one sense, the additional leap required for a correct understanding was very small. In another sense, it would have required a kind of Copernican revolution in their thinking.
I find this fascinating: the application — almost — of the scientific method, only to fail at the crucial falsification stage. And how the wrong answer propagates because of this failure.