This seems to start with two of my fellow players single (well, double-)handedly derailed the adventure our DM had planned
in the first session, but it actually goes back to the beginning. It's an adventure the DM had printed up from the 'net, so y'all might recognize it.
See, there was no shared backstory, no relationship between our characters, nothing. They were just all standing in the market when some kooky monster broke loose, and hey presto we're an adventuring party. Including a brooding Drow who hates clerics because she was supposed to be sacrificed to Lolth, a rogue/cleric Tiefling impersonating a local noble, and a Hexblade named Diabedes (I love my goofy names.) The characters were all very... antisocial.
But hey, we had a campaign to start, so let's get to it -- we're hired to investigate who, or what, is killing the gladiators. So we go undercover as gladiators, and we start getting stupid nicknames from the fat guy who runs the place. The Drow looks at him and says "Give me a nickname and die."
"I'll call you Stabs."
Poor Diabedes, being my first foray into something vaguely fightery, got himself killed within two hours of start by a Dwarf barbarian two levels below him. I decided to let him rest in peace and come up with a new character who might fit in as more of a team player... a Cleric of Death and Suffering.
While I'm creating my new character, and our poor good horse-riding barbarian does... something or other... the Drow who hates clerics and the Tiefling cleric decide to take matters into their own hands. After all, Stabs told the fat man he'd die, and the Tiefling is convinced that the fat man is behind the deaths. So, using some ungodly combinations of spells and skill levels, they sneak into his home and butcher everyone (except one servant) in the place. Everyone.
At this point, the DM looks from one player to another, looks at the printout, and tosses it over his shoulder. But we shall soldier on!
Fast forward through one session where nothing overly derailing happened, mostly because the DM really did toss aside that pregenerated adventure and switched gears.
This weekend, the barbarian's player can't make it, so we've got a Drow ranger, a rogue/cleric, and my fighter/cleric going on a dungeon crawl. Against a bunch of orcs. Not too bad. We get some flanking on one barbarian, things are going okay.
Except that the rogue/cleric doesn't mention when he's low on HP, and doesn't do anything about it himself -- and the barbarian rolls well. Not critical hit-well. But -12 HP well. And me one level shy of being able to cast animate dead.
So 3 sessions into the campaign we've lost two characters (both to 3rd level barbarians!) And with two of the players leaving at the end of the month, it looks like we're done.