For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
IIRC the orbit of Mercure doesn’t work with Newton Model, and astronomers were predicted the discovery of Vulcain a small planet between Mercure and the Sun. So a new model had to be invented since Vulcain couldn’t be found.