The Precession of Mercury
According to Newtonian physics, spacetime was flat. All the planets moved in ellipses about the Sun at one focus, and, neglecting the influence of the other planets and the non uniformity of the Sun – it is not a perfect sphere - the orientation of the ellipse is fixed in space. It was known since the early 1800s however that this was not true for Mercury. Even when the non uniformity of the Sun and the influence of the other planets was taken into account, there was still a discrepancy in the rate of advance of the perihelion of Mercury's orbit of 43 seconds of arc per century.