exasperation is part of the job
I have gotten a little rotten once or twice with someone after very long sessions, and felt like a total prick afterwards. Its always important that your student knows that no one is perfect even us instructors. Progress is all we can expect to see not perfection.
One thing that annoy's me though would have to be students that dont take their own time out to prepare and expect to be held by the hand all lesson. They should really be sent away to prepare properly and re-booked for a later lesson.