Canned adventures redux

So I went ahead and tried the whole "create a canned adventure" for the Session we are currently doing. I mean I basically had a good idea of where I wanted the adventure to go and what not, so I started typing it up. On the good side, it really helped me design a quest that used the strengths and weaknesses of the chars that are currently playing. However even with spending as much time as I did planning out all the possible outcomes, they went and did something I didn't even think about. Here is where I think the "canned" adventures throw me. When your GM'ing and you are doing it off of a vague idea, or a basic set of notes, you are pretty much prepared for anything that the chars throw out there, because you are not expecting them to do anything. However when you write up the way the story "should" unfold, if they don't stick to the (your) plan, it is easy to feel like a deer in headlights. That being said, I think I'm going to continue to create notes and basic ideas of how the quest should go, and layout combat (create monsters, villains, ect), but try not to detail every little thing, and have the chars do what they want and fit the story to them. Because after all, the story is about them, I just provide the backdrop :)