Hi Nick,
Nice question.
Something that frequently happens to me is that the students get interested in a topic and a conversation starts that is nothing to do with my lesson plan. In these cases, I always abandon the lesson plan. And - believe me - the lesson plan was always inferior to what the students made the lesson become.
Once, with a CAE class of Swiss and Japanese students, I'd planned to teach the Past Perfect Continuous. Instead, we had a two-hour discussion about the influence of the U.S. in the world. It was a great lesson, thanks to the students, and the Past Perfect Continuous was never even mentioned.
So, yes, my students frequently improve the lessons.
JJ