It wasn't a fuckup of mine, but a rather funny story about how much of a jackass I was when GMing back in high school.
I prepared a short (took three 4-hour sessions or so to wrap up), low-level D&D campaign for my three friends. I was supposed to GM follow-ups (had drafted two more short sessions and a finale that would supposedly end with the players around level 9) but then summer came, and after the summer we left high school and me and another guy lost interest in PnP due to whole new life starting at the uni and stuff like that.
So, anyway, the adventurer party reached an island. I wanted the story unfold slowly, so they didn't begin at a tavern talking to some seedy character, I just placed them at the port and managed to draw them into the storyline almost seamlessly. I achieved that by dropping quite a few loose ends at them which they believed to be "the real start". The best one was with the goblin conspiracy.
The party had to reach a larger force on the island for some mercenary work, clearing out a bugbear den that recently became a large problem for the local landowners, hindering their logging and farming enterprises. The merc recruiter gave directions to the party, some travel provisions and told them that the two best ways are to either follow the shore or take the road and turn south at a certain point. "Don't know which way is safer, to be honest. Strange things happen recently in the wild, best be on your guard while en route. Just be at the assembly point within a week. Sign this. If you don't show up you'll get blacklisted and won't get any more jobs with the guild anywhere, if you show up and desert you'll be found and hanged. This is everything you need to know.".
I noticed that the players got pretty excited at the notion of "strange things happening in the wild", expecting it to be a hint that something important is going to happen along the way. Both the route and the merc job weren't important storywise, so I decided to troll them a bit. They took the shoreline route and travel time would be around two days; on the second night the elf wizard passed his listening check and interrupted his meditation to notice a small figure creeping close to the camp. She tried to silently wake up the others but this time the check failed. Turned out it was a lone goblin who gave the leg immediately after being spotted. The party tried chasing and shooting him, but they only barely scratched the goblin as he got away, being the quicker one.
The party naturally imagined that this must be important, and the rogue, who was a fairly competent cross-class woodsman skillwise, attempted to track the escapee. They grew increasingly fatigued and got ambushed by wolves once (which they butchered effortlessly because of a really lucky roll with the wizard's sleep spell). They lost track a couple of times, but they still pressed forward. Finally, ten hours later or so, the tracks took them to the environs of an ominous looking cave with some shanty wooden huts around its entrance and lookouts which they barely managed to avoid. Adventure!
They moved away to a safe distance, set up sentries and camped for a well-earned rest. All details aside, the brave adventurers crafted a bold plan to assault the goblins. First they executed the lookouts, then butchered some goblins in their sleep, then entered the cave. It turned out it was fairly small, with a relatively tough fight of them against two goblin shamans and three ordinary goblins versus them - wizard, rogue and the reader, a special very jack-of-all-trades class I created for the campaign that could spend perks to develop in multiple ways but never could truly excel at anything. Through nice use of tactics and resources at their disposal - a good bunch of players they were - they managed to gut the goblins without any serious injuries. The looting started; they found a wooden box containing the goblin tribe's "treasures": a mundane but somewhat valuable gem, a long dagger with a cheap heat radiation enchantment and a mysterious parchment with some writing in an unknown runic language. The party debated what to do and decided to ignore the merc assignment and get the writing translated somewhere in the town they started out in, probably imagining that whatever is written there will foreshadow a dark plot or another that they are supposed to unveil. After leaving the cave, however, it turned out that they were lost - the tracks they used to reach the camp were gone, the forest was very lush and hard to get around in and they forgot to purchase a map and all three failed the relevant rolls. Thus began the six day-long odyssey towards the civilisation; they figured that following the goblin must've took them north while it took them east, and since they headed south they ended up in the most remote and sparsely inhabited part of the whole island. Still, they reached the shore, and following the shore they reached the city - badly bruised, malnourished, with torn clothing and damaged equipment (the reader even had to improvise a club at some point and the rogue used his bolt quiver as firewood), two of them with scarred faces.
I'm getting bored with writing all this, so let's cut the story short - after paying for the translation it turned out that it was simply a paylist from the neighboring logging operation ran by foreign workers. The disbelief followed by extreme butthurt on the players' faces was awesome - the session was basically over and they realised they wasted over four hours chasing a red herring, two of them received scars that permanently lowered their CHA and they wasted a job opportunity, blowing their standing with the merc guild. After restocking and purchasing new gear to replace the battered oldies it turned out that the profit from their grand escapade was just thirty coppers, a cape fashioned from wolfskin, a poorly enchanted dagger and a single experience level for the rogue.