If I don’t every 2 or 3 chapters find myself saying, “Wow! I sure didn’t see that coming!” then I feel I’ve failed as a story teller. – Which is one of the main reasons I never outline.
And any time I hit a spot where I get all paranoid that I may never be able to work my way out of this particular twist, I just remember, “Hey; I’m the guy who killed off his narrator 2/3rds of the way through one of his books without any idea whatsoever how I could ever finish the story after that.” I tell myself, “Heck. If I was able to work my way out of that one …!”