So today I had this happen.
During champion selection when bans were happening, first player to pick says "I'm going Nid and going top". I was second to pick so I ask if it's alright if I go Annie and take mid. 3rd and 5th player say its fine and say they'll pick Shen and jungle Warwick.
So it looks like we have a decent comp forming but then it gets to our 4th player, he immediately goes "Morons we need AD", picks Sivir and locks in. Which is fine, but then then for the remainder of the champion selection he criticizes rest us for our picks.
Moving on to the start of the game, I buy my items and start moving to the bush between blue golem and mid. Sivir goes "I'm taking mid", runs next to the mid tower and stays there. Nidalee says that Annie and I have already agreed on taking mid and top solo and you should go bottom lane with Shen. Well, this guy wouldn't have any other way and responded saying how we all probably suck and he needs middle more then anyone else. Word tossing happened between my teammates and Sivir so I just put him on ignore to try to keep myself IQ from going lower.
My question is, what do you do when your entire team agrees on where everyone goes but one player spontaneously comes out and takes your lane?
I ended up going bottom lane with Shen. Nidalee dominated top and Warwick gave us few kills at bottom, we won the game and Sivir ended up being the weakest player on the team.