“The great hulk of the scaled beast lay wheezing before me, and I am not ashamed to say that I was frightened: though it was injured, I knew the dragon could easily tear me in twain. The young monk, however, knelt beside the creature’s head, whispering to it in a strange growling tongue. I questioned the woman, but she ignored me completely, engrossed in examining the scaly monster. After much badgering, she said merely, ‘I do not mean to be rude, but you are quite a distraction. Please, leave me to my work.’” -Jeron’s Great Journeys, Book III, Chapter 4
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.