This caught my attention recently: Backyard astronomer discovers black spot on Jupiter - Times Online
It looks like another collision: Scientists at Nasa confirmed that his observations were of an impact rather than a storm. It is thought to have been caused by a small comet or cometary fragment, about 1km in diameter, which would have struck the planet at a speed of about 60km per second (about 135,000mph).
This is the second time in 15 years that a comet has struck Jupiter. The gas giants perform an important role in 'sweeping up' stuff like this instead of it being a danger to us.
But the worrying this is that we never saw this one coming, and even if we had, could we have done anything about it if it was Earth bound?