As someone who has been operated as a day-to-day scrum master for over five years, I have had the pleasure of working with over a hundred people in a Scrum setting. Their prior knowledge about Agile and Scrum ran the gamut from those who had never heard of Agile or Scrum to those who have been a part of a team at an existing place of employment. If I had to pick which I would rather have on my team, I would choose the complete novice every time.
Reading in bed one night a wonderful book called The Invisible Gorilla and Other Ways Our Intuition Deceives Us. One of the interesting facts that was presented involved the fact that people who exhibit confidence in their knowledge are usually the least knowledgeable. In other words, true experts are more likely to not have a high degree of confidence in their assertions. I think that this is because the true expert realizes complexities that someone with just a little bit of knowledge would not even know they do not know. In my experience, nowhere is this more apparent than with people who have had some prior experience with Agile.