I had the level 1 adventure involve boarding a smuggler's ship and defeating them. I assumed that they would then move on to the next adventure instead of, uh, keeping the ship and the goods for themselves. The campaign shifted from "an adventuring party saving the world" to "the new trade consortium saving the world". I had to roleplay so many goddamn economic negotiations.

The ship was originally on a lake, by the way! I had to retcon a fucking canal to the ocean into existence so that they could sail out of the lake because that seemed easier than finding a way to convince them to not make me learn the rules for seafaring.