According to this BBC news story, it has been known for ~20 years that planetary orbits are not entirely fixed, but show slight natural fluctuations. Recently, astronomers J. Laskar and M. Gastineau have developed numerical simulations of the evolution of the Solar System. In ~1% of these solutions, the eccentricity of Mercury's orbit increases to the point of allowing collisions with Venus or the Sun. Still more alarming is the scenario in which a subsequent change in Mercury's orbit destabilises all the terrestrial planets, with consequent collisions of Mercury, Mars or Venus with the Earth.
This "complete devastation" only arises in a tiny minority of simulations, and wouldn't happen for several million years, but the possibility is nonetheless disturbing...
Nature: Existence of collisional trajectories of Mercury, Mars and Venus with the Earth