Yeah, it's very unstable at first. You're just bursting with excitement that you can change this dream into ANYTHING, that you lose control. Trust me, this has happened a lot. :/
A little tip to get control more frequently of dreams (when you didn't plan it). Look at your hands every day to see that you have 5 fingers. If it becomes a habit, you'll do that IN the dreams as well. And in dreams, you NEVER have 5 fingers. That is how first got control of my dream. I remembered INSIDE the dream that if you have more fingers, then you're dreaming.
And voila it worked. But the dream ended as I got excited