There's this interesting story: Did NASA Accidentally “Nuke” Jupiter?
Long story short:
On September 21, 2003 Galileo is deorbited in order to prevent eventuality of contamination of Europa. It descends into Jupiter atmosphere.
October 19, 2003 an unexplained "blotch" appears on surface of Jupiter.
The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much the plutonium went supercritical. The plasma bubble takes some time to expand and reach the surface, producing the spot.
How likely is this scenario? Is this just a hoax, a baseless speculation, or is this a fact? Can someone confirm or debunk this story?