Admirals, I present to you the future of space warfare! No longer will you need to waste billions of credits on costly space battles, and millions of lives on planetary warfare with hostile cultures. For only a fraction of the cost of a heavy cruiser, I present you with a weapon capable of eliminating an entire solar system. "I present the Star Hammer!"
(A powerful and deviously simple weapon to make space warfare in any sci-fi game forever changed)
The local band of 'nasties' (goblins / orcs / whatever) lives in relative peace with the local population.
Along come the PCs and go through their usual heroic monster bashing routine, wiping out the nasties and pinching all their stuff, then continue on their way.
Problem is, they don't kill ALL he nasties. The survivors want revenge and, after spending a few months recovering, start to take it in their own inimitable style (which is not unlike that of the PCs come to think of it).
Next time the PCs are in the area they find themselves VERY unpopular with the townsfolk.