Answer:
The only documented cometary strike occurred in July 1994 when comet Shoemaker-Levy 9 struck Jupiter's southern hemisphere. The comet had been ripped apart by Jupiter's immense gravity on a previous flyby. A sting of 22 fragments ploughed into Jupiter. Some of the fireballs erupting from the cloud tops where as large as the planet Earth.
All planets and moons have been struck by comets, and in all likelihood will be struck again. The Sun is hit by comets more often than any other body in the solar system.