Obviously I cannot run an adventure from the main campaign, as it would be completely unfair to the players that could not make it out for the bonus session: "Wait, you ran a session without me and you managed to sink our ship!?"
During the regular sessions, the PCs are captain and crew of the privateer vessel The Blacklass. The session prior, the crew had made a stop at cloud-island of Crail for a few supplies and alchemical ingredients. I thought the obvious thing to do was to let the players play the until-now unmentioned members of the crew on their shore leave at Crail.
By this time, the players were familiar enough with the system that tossing together some new characters took less than ten minutes and they were ready to go. I however was not. I hadn't found time to prepare for this last minute session, and had zero plot beyond having the B-Team on shore leave. So I needed something quick that would spur the players to action, something that the players could shake their fist at me for.
So the session is starting and the only thing that comes out of my mouth is: "So you have all contracted genital warts." followed by blinks and stares and nervous laughter. And then the adventure began. The cloud-island of Crail is known as the crossroad of the seven skies. If there was anywhere to find a cure for genital warts, Crail is the place to do it.
Quickly the players had split up to track down the ingredients for the cure. At this point, the game was going in the right direction, but I wanted something more from this session. I wanted things to be worse on the characters. I quickly came up with a mechanic to allow the players to bestow misfortune on the other players.
A PC was trying to flirt information out of a sailor when one of the other players throws in: "Its a well known fact that genital warts have a very distinctive smell. You reek of genital warts." And from here the game descended into brilliant madness.
For the players that made it out that night, this session is still the most talked about out of the entire campaign. All it took was genital warts and a little bit of friendly antagonism.