I have commited a slight act of DM-Dickery in the past, specifically the last session my group played.
I opened with the setup of some dozen or children going missing, all from towns near a castle where a vampire was supposed to live. Now I mentioned in the back-story for the vampire that he threw his lot in with a demon, becoming said creature of the night, after his children died of a rare disease, and in a fit of grief, his wife killed herself. This promptly drove him quite mad. Now throughout the castle I left plenty of hints about the nature of this disease. It doesn't affect adults. Comes around once every few generations etc. Now you can probably see where this was going.
Now that party had obviously been employed to rescue the children as in my setting, you can 'cure' someone of vampirism by killing their sire before the first full moon, which the party did obligingly. It was only then that they found the children, all dead from said disease. It turned out they had all contracted the same disease as the vampires children all those years ago and the vampirism was holding it at bay. The party had been pretty much screwed from the start.
But they rallied magnificently, and in a pure 'Single Manly Tear Was Shed' moment, appologised to the town with a single, heart felt (diplomacy natural 20) "Sorry", giving up all their gold and even planting a rather powerful banner in the ground as a memorial to the children and the vampire who had only been doing what he throught was the right thing to do.
In the same session I also managed to fuck with them after they entered what seemed to be a childs bedroom and listed the contents; "A bed, a dresser, wardrobe, a little girl, a bookshelf and a chair". When they asked about the 'little girl', I vehemently denied ever mentioning such a thing. Drove the party mad and they didn't spend too long in that part of the castle.