I had such a fortuitous event in a campaign the other day, I had to write about it.
One of my parties has been climbing steadily into the back country of the Albanian Mountains, and had reached the town of Ohrid on the lake of the same name, on the border of modern day Macedonia. I had conceived of the town as a center for the study of thaumaturgy, music and ecclesiastical magic, and I had conceived of a trick to pull on the party.
When the party arrived, the town was having a festival. The party were permitted access through the gates, and were encouraged by the townspeople to drink and be merry. After having sold the magical nature of the town, the party was asked repeatedly if they had "tried their luck." Everyone in the town was trying their luck, which was an event where each person waited in line to turn the wheel on the town's magic fountain. The fountain was huge, and it measured - so they said - the amount of actual luck the wheel-turning individual actually possessed, or would possess for the next year.
The party naturally got in line.
As each person ahead of them turned the wheel, the fountain produced differing amounts of water, which quickly dried up. It was clear the fountain was magical. People around the party talked about their luck last year, and I described how some people were moderately lucky, and some were not lucky at all (no water). There were six tiers to the fountain, and no one ahead of the party filled better than the third tier.
The party got their turn; I had them roll a d20, then turn the crank and measure their luck. I explained that the second person's luck was determined by their own roll, and the first person's (since they were in the party together, the other members of the party who turned the wheel before affected the luck they had).
When the last party member went, they rolled a 1 on a d20. The fountain turned into a geyser, the town went mad with glee, all the tiers filled to overflowing and the luckiest person in the party had been found!
It was SO convenient that the last player had rolled a 1.
A foot pedal controlled the fountain. I had planned to make the fountain roll and flow at full volume no matter what the last player rolled - the town was simply waiting for someone tough and capable (and foreign) to step up. Afterwards, the pasha of the town wined and dined the party, gave them whatever they wished for (including magic potions), gave them all the information about their journey they could wish, and asked for just a little favor - could they please join a group of three magi descending into an oracle to give the beast within a present. The luckiest person in the world, surely, could keep the three magi safe.
In the middle of this sales pitch, the party stopped and laughed to themselves about how well I was role-playing the pasha, how interesting it all was, what they were going to do with their good fortune, etc ... while I managed to keep my face as straight as necessary. Loaded up with potions, the party descended into the oracle with the magi, who then vanished as the doors above were closed. For the party, one townsmember sacrificed himself to try to inform the party, so the party learned - too late - that the whole thing was a ploy. The potions were not potions, the fountain did not measure luck (as I said, it operated via a foot pedal, secretly used by the overseer of the fountain), and the party was screwed.
Was it fair? Of course. I absolutely figured I would fail to convince the party to go down the well that was the oracle - and then one of the party members rolled a 1. Ah, the magic of numbers. After the 1 was rolled, it was easy to believe everything the pasha and his court said.
The world is full of nasties.
So the party managed to determine their in a dungeon with a minotaur (who they saw, but haven't met yet), and a group of lepers. Fun times.