As the small group of mercenaries settled down for the night, they failed to notice that some small pieces of barbed wire sticking out of the ground nearby were moving like tentacles.Until it was too late and they woke to feel it clutched around their throats and squeezing hard. As their blood spilled upon the ground the Barbed Arms had claimed yet more victims.
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.