Last week, I posted about writing what you know, and it's funny how the things you know can crop up without you knowing it. :)
I read through some of my YA novel yesterday, and discovered something about the parents of my main character (Rose). Their relationship is exactly like the relationship my parents had when I was growing up. Rose's mom is nothing like my mom, and Rose's dad is not like my dad. But the way her mom and dad interact with each other is very similar to how my parents interacted when I was growing up.
So, I took a piece of my childhood, something I know very, very well, and unconsciously put it into my story. I'm sure I'm not the first person to have done this, so it makes me think that we all know more than we think we know. :)