Joe, it happened to me at SRU. My prof was extremely liberal. I had completed all the required course work with straight A grades. I never missed a class, test, or assigned work. Yet, during a class debate a argued my rather centrist point of view with that professor.As i was walking out of the room that day, he stopped me, smiled and said, "you just earned yourself a "C", and go right ahead and complain if you like, i have tenure".

A very similar thing also happened to my best friend, who went to PSU. So yes, it does happen and i think more often than people might think.