A recent email from Mike Cohn covers the top 10 mistakes ScrumMasters commonly make:
Trying to facilitate meetings using someone else's style
Holding meetings or events at the wrong time
Telling people what to do
Staying Silent
Not asking enough questions
Forgoing the usual meetings due to team size
Not 'pushing' the team a little harder towards improvements they commit to make
Allowing too much solutioning in the Daily Scrum meeting.
Letting a story into a sprint without enough clarity
Not letting go of mistakes