The Death Star II in orbit around the Forest Moon. Even if the station wasn't around the size of our moon, it would still be enough to cause a worldwide catastrophe.
During the Battle of Endor in Star Wars Episode 6: Return of the Jedi, the Rebels destroy the second Death Star in orbit around the forest moon of Endor. The destruction of the battle-station kills the remaining Imperial forces on board and delivers an enormous blow to the Empire due to the loss of such a gigantic weapon, the defeat of the Imperial fleet nearby, and the death of the evil Emperor Palpatine. And our heroes are celebrating at the end, all fine and dandy now that Darth Vader has been redeemed. Add the Special Edition and you get celebrations on numerous worlds across the galaxy against the Imperials. Sounds fine, right?
Consider that the Death Star II is around the size of our moon. When the station erupts in a gigantic fireball, numerous parts of its frame and inner workings are ejected outward into space at enormous speeds. Also consider that in real life, when a space station that is vastly smaller than the Death Star II is destroyed, it creates a cloud of debris that begins to circle the Earth due to being drawn in by the planet's gravity. If the debris collides with satellites, it can cause great damage to them.
Imagine all that debris, much of which could be country-sized, falling down onto the Forest Moon. The gravity field would pull on the wreckage so that it would be shifted towards the Moon. The gas giant Endor is directly behind it (the Forest Moon) and thus cannot pull enough of the debris away. The Rebel Fleet is too small to create a combined shield to deflect even half the stuff. And the Imperials, even if they were helping the Rebels deflect it and had all of their ships, would not be anywhere near enough to protect the moon.
The resulting impacts would practically annihilate the biosphere of the moon. Enormous clouds of dust would be thrown up into the air, blocking out sunlight for many miles. This would be in turn supplemented by forest fires caused by burning wreckage strikes. The native population of the planet, the Ewoks, would have to be evacuated off-planet if they were to have any chance of surviving this catastrophe. It is doubtful that they would; the Rebels are strapped for ships and could evacuate only a mild sliver of the Ewok natives.
This would not be an unlikely event - one lone asteroid was enough to cause a global cataclysm that wiped out a great deal (if not the majority) of the fauna and life on Earth during the age of the Dinosaurs.
Thus, the great victory at Endor would come at a great price - the holocaust of nearly an entire population.
Original author unknown
Originally uploaded on March 5th, 2012
More information on this theory available here.