Oh, my. I've got a few.
In the campaign, the party is following in the footsteps of a 112-year-old explorer's last voyage. The explorer was the only one to come back from his last voyage; all 12 members of his team, the crew and captain of the ship he chartered, and the ship itself, vanished. The explorer refused to reveal any information on what happened, when he got back two years later. After living as a recluse for approaching five years, he took his own life.
Fast forward over a century. The explorer's great-grand-son--the explorer had children before his last voyage--hires the party, on his deathbed, to find what it was that marked his great-grandfather so, discover what he never wanted found.
They went off in a ship, did a bunch of stuff, a couple of them died, and they got to the island where the explorer said he washed up. Bad things continued to happen, horrible, many-eyed-and-tentacled monsters abounded, and they actually found some remains of the previous team of explorers, including one petrified half-elf fighter. The dice were with them on their adventures before arriving at the island, and they had a potion of greater restoration, so they used it on the petrified half-elf. He was grateful, of course, and answered their questions as they escorted him to the ship.
However, something fun was revealed. The party said, "Oh, we were hired by your old leader's great-grandson."
The half-elf responded, confused: "Great-grandson? The leader had no children, and was infertile."
----complete, dead silence----
The party: "What?"
The half-elf: "The leader was born that way, and the various clerics were unable to cure him of it, when the time came. That's why he became an explorer--his one failed marriage because of lack of children led him to attempt to immortalize himself with his discoveries."
The party: "WHAT?"
Up until this point, no one had even questioned the "great-grandson's" motivations. Didn't even think to ask.