A great example would be Linus Pauling, who later on in his career became a proponent of mega-doses of vitamin C as a cure for all sorts of stuff, including cancer. He remained obsessed with the idea in the face of repeated studies that have shown it's no better than a placebo (in fact mega-dosing may even increase your risk of some cancers).

History is full of really smart people believing some pretty stupid things, especially in areas outside of their area of expertise. We have a very one-dimensional view of intelligence that assumes that because someone is smart (or even a genius) in one way, or knowledgeable about one area, that this must automatically apply to every facet of intelligence or knowledge.

Smart people are still people, with all the same biases, preconceptions, ego and other human frailties.

For some reason some physicists in particular seem very prone to underestimating the complexity of other branches of science, and over-estimating the applicability of their ways of working to them, especially the biological sciences which are full of complex systems that often turn out to react in unexpected ways.