[3] Centaurs are small bodies with unstable orbits in the outer Solar System that cross the orbits of the giant planets. (sciencedaily.com)
However, there were bodies exhibiting unstable orbits that Newton could not explain mathematically with his models. (religion.blogs.cnn.com)
The authors also note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, without the help of orbital resonances. (sciencedaily.com)