George Hall, an amateur astrophotographer was observing and recording Jupiter last Monday (10 Sept. 2012) and he happened to capture what appears to be something colliding against Jupiter. Apparently another astronomer also saw the event, and NASA officials say that they can only confirm the collision based on these sighting and official reports are pending.
Quoted from Amy Miller from NASA:
Although we don't yet know the size or exact nature of the impactor, based on the flash brightness we expect it is slightly bigger and energetic than the one seen in 2010, which was estimated to be on the order of 10 meters [33 feet] in size. By contrast, the impactor in 2009 was likely 200 to 500 meters [660 to 1,600 feet]