My worst is easy to remember, because not only did it destroy my campaign, but several friendships as well. TL;DR I introduced an NPC so divisive that it broke the players apart.
I ran a Legends of the Five Rings campaign that was my first attempt to go of canon, and I told my players this. The gist of the campaign is that the party is carrying/guarding a powerful katana that is magically bound to its bearer. The sword contains an oni (demon spirit) that's prophesied to be released and lay waste to the land. A different prophesy (unknown to the party) states that a kami (divine spirit) is bound to the crosspiece, and a powerful shugenja (mage-priests) from each element would work with the Final Swordbearer to awaken it, cleansing the sword and empowering the Final Swordbearer to be able to destroy the oni forever.
The problem occurred when I introduced the first elemental shugenja, an Air shugenja as cold as the mountain air. She was intended to give off a bad vibe to the party (she was evil, but not EVIL), but she was to be considered a necessary evil. She's the one who brought the prophesy about the oni, implying that she's going to help bring it about (since it was necessary to have the second prophesy occur). Everyone, especially the swordbearer of the time, was very worried about her, preparing for how to deal with her the next time she appeared.
She appeared again several months later, this time saving the party's life, and before the party could act, she summoned a magical bubble around her and the swordbearer. Within, silenced and hidden from the rest of the party, she explained the second prophesy and insisted that this information was not for outsiders (the party). She then vanished, ending the magical bubble. The party asked what had occurred (as we were away from the table), and he said that they were wrong about her, and that she believed in her. The party demanded more information, and the player refused to answer. Convinced he was enchanted, the subdued him, which ended the session. I thought that this was going to be a fun next session.
At the session, the players interrogated the swordbearer, who refused to answer any questions, claiming they had to trust him. Lacking information, and already terrified of the sword and shugenja, the party decided they couldn't allow them to meet again. The entire session afterwards was them planning on how to keep them apart, even considering a way to kill him without allowing a new bearer to be chosen. Since no action was being done, I just sat and listened, figuring out how to counteract their plans. The swordbearer was getting really upset, especially at two other players that were his really good friends, as they had the most radical and extreme ideas. No one was listening to each other, and the session ended badly.
The following week at our other game (we alternated between L5R and D&D), things were tense. I didn't understand, and talked with the DM afterwards. He was one of the three close friends, and all three had been fighting over the phone during the week. Apparently the swordbearer player wouldn't explain anything even out of character, falling back into the "it's not for mortals to know." I was shocked, totally expecting the player to give away the information under such real world duress. The start of the following session I ended the campaign. No one was having fun, and it was having real world consequences. I explained my goal, and everything that was going on. Everyone was furious at me, which I was fine with (better me than each other). I took a break from GMing for a while, but those three were never close again.
In hindsight, I should have introduced one of the other shugenja to give some context. Even if I hadn't, the session they planned I should have put a stop to it, explaining what was going on. The campaign might have been ruined, but at least I might have saved those friendships.