I've had, and seen as a player, many campaigns fail to work out for various reasons. The funniest/sadist one I was a player in ended before the end of the first session. There were 2 power-gamers in the group, and they competed aggressively against each other all the time. To attempt to head this off in a 3E D&D game, the DM had everyone roll in secret, with only him knowing the results, so that no one would know how powerful the other characters were. During the first session, one of the power-gamers was annoyed at how much better the other one was doing, and snuck a peek at his sheet while getting up for a drink. He pitched an epic fit, called the DM a cheater and helping out the other guy, because he'd rolled 18 twice, and got a third with a 17 and a racial bonus. It's important to note that I only game with 1 player from the group anymore (neither power gamer or DM).
My own epic failure of a campaign was so bad it ended friendships. There were 3 players who were very good friends, and they tended to stick together whenever there was tension in the group (two of these were from the group above). The campaign's focus was on a sword that had the spirit of a demon trapped within it, which bonded to a character. The demon could influence the bonded character, but PC caliber characters could resist most of the effects. Before the character could die, they had to pass the sword off to a new bearer. They knew that if the sword ever fell into the hands of an evil person or a weak person, it would take them over and bring forth a prophesied apocalypse, so they kept transferring it among the PCs.
The goal of the campaign was to find a way to kill the demon within the sword, and they eventually learned that 5 NPCs representing the 5 Elements (normal four plus Void) would need to be brought together to do it. The first NPC, however, was immediately disliked by most of the group, including 2 of the 3 players mentioned (the 3rd was the bonded character). Because of this dislike, and betrayal and treachery being a common aspect in the game, the distrusting players began investigating the NPC, including using divination magic. They discovered that she planned to release the demon during the ritual... but their questions never asked why.
They informed the bonded character of their discovery, who was now in a romantic relationship with the NPC. The bonded character immediately went to talk with the NPC alone, because he knew that at this point everyone else would be hostile, and from everything he'd seen, he didn't believe it was true. She admitted it was true, but only because the demon had to be freed before it could be slain. She decided to avoid the party until the group had gathered the other four NPCs, because she didn't want to create animosity. When the bonded character returned and explained everything, they didn't believe any of it, convinced she had an ulterior motive. The discussions became heated, which continued even away from the session since the 3 got together regularly.
They almost came to blows over it, and I simply called the campaign over in an attempt to smooth things over. I didn't want to continue discussing it, but was eventually pressed on the issue. I admitted that yes, the demon did need to be freed to kill it, and yes there was a traitor among the 5 NPCs... but it wasn't the one they'd met (remember, they'd only gotten to ONE of the five NPCs at this point). The 4th NPC was going to turn against the party once it was freed in order to serve the demon, but that was WAY in the future of the campaign. Instead of accepting this, both sides assumed they were correct (the distrustful ones thought I lied to keep the peace, which TBH, I would have done) and the friendships died. They'd still game together, but as no one was willing to back down, they were never close again. They no longer game together, but it's still a sore topic for all three, even though this took place over a decade ago.
