I don't think I could have held out for twenty minutes in the waiting room. I think I might have left after fifteen. 10 minutes for me is excruciating!!
A similar situation happened to me once with ex-T. She only left for 5 minutes, and left me in her office while she went out to the waiting room to handle a client that wouldn't go away. She also apologized profusely before and after. Our time ran over that extra 5 as well.
I think what your T did was un-professional and un-kind.