Sometimes, if I am really in the zone writing, something will happen completely out of left field that I had no intention of putting in the piece. Do that ever happen to you?
For example, many years ago, writing "Grandmother Clause" (one of the short stories in RieVisions), the main character buys a potion that will give his Grandmother several more years of life in exchange for some of his own. She refuses it, and tells him to pour it out. As he goes to obey, he meets a little girl in the hallway who is suffering from a terminal illness and has come out of her room for a drink of water. And he gives her the glass with the potion mixed in it. I had no idea he was about to do that until he handed her the glass. It was an in-character thing to do, but it was not something I had planned for the story.
Another example is my upcoming book, The Luckless Prince. When I first started writing it, all those many moons ago, it was the story of Roland and what happened to him. But as the book came to life, the character of Stefan became more and more compelling, and he is now the main focus. In this case, it isn't so much any one detail, but many little actions that were surprises.
What about you? Do you ever get unexpected dialog whispered in your ear? Do characters suddenly turn down a path you meant to pass by? Do you listen, or do you rein them in? What works best?