I have a tendency to borrow books from people and then move to a new city / country / continent without giving them back. I don't even do it on purpose, I just forget about it until it's too late.
There's a trick to that. Take a Post-It and write their name on it, then stick it in the front page of the book, sticking out just slightly at the top. That way you can easily know "this is a borrowed book" and then open it to find who it belongs to.