Hi All,
I'm a test consultant that has worked in several organisations that have tried to do Scrum. (it was never perfect)
What I've noticed on 3 occasions is that a long term scrum master becomes a sort 'manager' of the team. With all kinds of consequences: SM becomes frustrated if team doesn't deliver, SM places himself outside of scum team ("I'm happy with the amount of work you delivered"), acts like a manager ("I will not accept that user story"). etc.