In Wikipedia they mention that the Cassini spaceship will be destroyed in by sending it into Saturn's atmosphere:
However, due to the spacecraft's dwindling fuel resources for further orbital corrections, it is currently planned to be destroyed by diving into the planet's(Saturn) atmosphere in September 2017. This method of disposal was chosen to avoid potential biological contamination of Saturn's moons.
This is to make sure that it will never crash into one of the moons (like Titan) where biological material would contaminate.
But how can a spaceship be a source of potential biological contamination?