Twists

So I've been writing a lot. In this one story in particular, I was pretty sure I had everything plotted out. Then suddenly this new idea came and I realized it was the one I needed to make everything fall into place. Its happened with characters too. Characters have started up one way and then ended up completely different from how I imagine them to be. Its one of the many things I love about writing. So tell me do you like when your story takes you completely by surprise or does it frustrate you?