All this talk of snowflakes and ponies has brought back memories of some of the more cataclysmic derailing i've had occur in games over the years. One game I played in years ago we had a rogue who was working as a bounty hunter/assassin. The party was unaware that he had "dead or alive" contracts on some of the NPCs. We were all set to interrogate a prisoner for information about the MacGuffin and out of the blue he stabs the NPC, killing him with a coup d'grace.
What's the DM's roll in games where there's intra-party conflict. The DM liked all the players, he approved all the characters. Should he have been a peace maker between the players? Should he write narrative feeding the assassination storyline? What do you think about intra-party conflict in general? Do you have it in your games? Do you enjoy it?