The moment the teacher realizes it was never about the perceived correct answer, but the questions that led to the paths taken. The sudden realization that teaching is more than the teacher initially perceived. Its not about teaching "this is so", but rather, "why do we know this is so?".
Which is what education should have always been about. It's not about responding with the correct answer. It's about asking the right questions. A famous Greek philosopher knew this, as did many before and after.
This is another after.