I never realized how much I said "I understand" to people until I saw an episode of
Two and a Half Men wherein Charlie explains to Jake that if you want to sound sympathetic to a woman, all you need to say is "I understand." Now every time I say it I do it with a tiny inner giggle.
I've always worked on trying to avoid certain repetitive phrases as my major in university required a lot of public speaking--plus, I'm just the kind of person that can't be quiet in class.
The one that drives me crazy is "like." If I ever taught a class that required kids to speak on a subject, and one of them got up there and started saying "like" every other sentence, I would stop the speech. Of course, I would warn them of this beforehand. It was the policy of a couple of my high school teachers--"like" and too many "um"s were not to be tolerated.