You can plan every day of your life up to the minute if you so please. You can have guides, maps, and knowledge of your destination. But things change.
Change is the framework of life.
I'm a planner. Always have been, always will be. I prefer to have dates set months in advance. To know exactly, without a shadow of doubt, what will be happening, when, and where.
My sister and I...okay, so mostly I have been planning a trip back to the Wizarding World of Harry Potter in February. It was going to be the trip of all trips. Super Spawn is getting tickets and traveling things for Christmas. A portable DVD player, a pillow pet!
Then I learned that I'll be having Super Spawn opposite weekends of what I had planned. Because other people don't plan, and adore making things difficult for me. *glares*
Anyway, now we're going for the New Year! Why not? It'll be fun! They'll still have all the Christmas decorations up! It'll be cold...like it's supposed to be in Hogsmead. We've already got tickets and everything. So...it all worked out.
And the same thing applies to your writing. You may start out with a story in mind. A particular path you want that story to take, or a particular thing you want your character to do (or not to do). Sometimes though, even with the best intentions, things change. That character may develop a mind of their own, wander off into uncharted territory, and bring you the best story you've ever written.
Who's to say it couldn't happen?
So, keep your minds open to change. Be accepting of it in your life and in your writing, and something good may come of it.