Sometimes, deep into a manuscript something happens. One of the characters suddenly does something that you, the author, did not expect. Some people attribute this to a muse or divine intervention. Stephen King spoke of a guy coming up from the basement and depositing something on the table in front of you. Jennifer Crusie often refers to the girls in the basement, the idea that no matter how much plotting or outlining you do there's always a moment when your fingers put down something and you go, huh, well, that's not quite how I had planned that but interesting. And still others talk about how those ideas rattling around in the back often create something (it turns out they're brothers) that ends up adding a new layer or wrinkle that's really kinda cool.
So, I was pointed to a story from this summer. This couple were on vacation, and they wanted to take one of those - Here's us in front of the lake - photos. And they setup the camera and set the timer and got into place, and just before the picture was taken a squirrel popped up intrigued by the sound of the camera. So, their picture ended up looking like this.
So, sometimes in writing you end up with surprise squirrels.