Ran into a somewhat game-breaking bug last time I played; during the "Atonement" mission where you meet John at the church in Fall's End. After you get outside and go to drive off with the AI driver, each time I died in that vehicle segment (which totally sucks by the way), I would re-spawn right as I got into the turret...
...but what's her face, who is supposed to drive the damn truck, spawns all the way across town and by the time she finally jogs over to the truck the mission times out and is forfeit and I can't progress because it will just keep looping. The only thing I can do is quit to the main menu and restart the mission if I so choose. There is no option to quite that particular mission whilst it's in progress. Annoying.