In an AD&D campaign I ran several years back, the party was suppose to go to a farm and investigate what was happening there; they discover a small orc group ransacking the farmstead.This being a low level adventure the orcs outnumbered the party two to one. I figured the party would do guerilla tactics to take out the orcs. Boy was I wrong.
One of the party members said “Well this ain’t our problem”. So, he convinced the others they should tell the local road wards and they left. Here I had prepped that evening for that particular adventure and now I had to go on the fly and make things up, taking notes as well, as I went along. The party went 180 degrees.
So they went off to a town to see what else was interesting in the area, they check the local job boards; missing child in the Sandifoot Hills, An old temple ruins discovered near the Dracos Mountains, etc… They talked about what they were going to do, bought some gear, and decided to proceed on the next adventure.
Thats when the town they were in was attacked by a horde of Orcs. The orc scouting party discovered a weakness in the Kingdoms defense, because the party forgot to inform the road wardens and the local law about what they saw at the farm. So the orcs didn’t meet any resistance at all so they reported back to tribe and viola. The campaign became a war story, and the players went through hell for several adventures; short on food, broken weapons, no mending their armour at all. It was one of the most brutal campaigns I ran.
Good times, good times.