A recent email from Mike Cohn covers the top 10 mistakes ScrumMasters commonly make: 

1. Trying to facilitate meetings using someone else's style
2. Holding meetings or events at the wrong time
3. Telling people what to do
4. Staying Silent
5. Not asking enough questions
6. Forgoing the usual meetings due to team size
7. Not 'pushing' the team a little harder towards improvements they commit to make
8. Allowing too much solutioning in the Daily Scrum meeting. 
9. Letting a story into a sprint without enough clarity
10. Not letting go of mistakes