This is for the ones who write their own campaigns, for the most part.
When you start out with a clear goal and set of circumstances you wish your players to deal with, how do you plan for the things that players do that surprise you and change what you have already made provisions for ? In my campaigns, there was always someone who decided to act in a way not planned for and I would either have to adapt my campaign or try to guide the errant character back in the direction I needed him to go. Neither was easy at times, but doable. Has anyone's character ever ruined a campaign beyond saving ?