Yes! When we did the antagonist background story, I had a vague idea going in (my idea was still very tentative). My antagonist literally wrote herself into the story. She did NOT exist before I sat down to write.

I then spent 3 days scrambling, trying to figure out what just happened, completely overhauled my outline, readjusted characters to fit the new plot... It was chaotic. It also changed the tone of my story and the age. Instead of being light-hearted (my usual tone), it's a little more mature. So I had to age my protagonist and I admit I'm a little nervous because she may have only aged a couple years, but the story has matured to a level I've never written before.

I'm still not sure how to get from point A to point B then on to point C. But I at least know what A, B and C are! All I'm doing right now is summarizing all the plots and subplots and hoping to thread them all together cohesively come November.