"Neither steel nor stone could cause any damage, my Lord... It was simply too big, unlike any enemy we have ever seen."
Legion Captain Meron's last report to General Lorgar
"When the trees shuddered and the earth shook, we knew it was far too late to retreat."
Excerpt of General Lorgar's last report to Emperor Paulus, following the loss of his entire army of 10,000 men
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.