I tried the Co-op missions last night for the first time and really enjoyed them a lot.
However, I was playing with this set of 3 people who also happened to be friends offline. We had gone through 2 missions and were attempting a third, The River, when one of the three got a little bored.
He had chosen the demolitions character style, but started by shooting his buddies in the head, and over the headset, I could hear the other two getting upset with him for doing it. To which his response was to cackle at them, naturally.
As we approached about the mid-point of the mission, he stepped it up and began throwing fire bottles at them on the front of the raft. I had managed to position myself behind this guy, so I did not get caught in it. But he managed to remove one of the other two entirely from the raft.
When we reached one of the bandit camps and cleared it out, I was the last one back to the raft, and this guy was already pelting his buddies with fire bottles again. One of them goes down and his life starts draining when I make it back to the raft and the next sequence loads.
When we show back up aboard the raft, the person who was down, was still lying on the planks of the raft, still ablaze. He couldn't get up or move at all. Eventually he 'died' again and the game was nice enough to re-spawn him aboard the raft.
We head down the river a bit further, and as I'm sniping some dudes aboard another raft, I see the trail of a fire bottle go through my vision just as I hear over the headset: "No! Stop! Don't!" and "You'd better not...". But I was unable to get behind the clown this time.
I do the little dance and burn and my body slumps to the planks and begins to slide off the back of the raft. As soon as I hit the water - all of my health was restored and I'm standing upright, but all I could do was run in place on the surface of the water:
I couldn't even move the camera to see what was going on with the raft. Eventually the game dropped me out of the session, and I had a hell of a time trying to get back into Co-op for a bit afterward.