In a bar not too far off the road, a man walks in. his boots clattering on the wooden floor. The bar is noticeably empty with only a few guys still around either drinking or chatting. Not too surprising since it is already the evening and they probably went home. The smell of smoke and liquor still hangs in the air like a fly caught in a web. The man in the trench coat pulls himself on a stool near the counter. His face covered by a bandanna and the brim of his hat obscuring the view of his eyes. The bartender, who had been cleaning a beer glass, looks up.
"So what'll it be, stranger?"
The man responds in a gruff and tired voice. "Dark and Stormy."
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.