Allow the team to Fail
What? That's crazy talk.
Why would I ever do that? What good can possibly come of that?
Standing by and watching someone fail is really hard for a Scrum Master. In fact, it's hard for anybody.
But it is necessary.
Simply put (in user story format):
As a Scrum Master I need to allow my teams to fail so that they learn from their mistakes.
It's just as important to learn how NOT to do something as it is to learn how to do something correctly.