I am a PhD student, and just gave a talk about a model that my advisor and I have been working on in a small conference. During my talk, a professor pointed out that our model will have an serious issue in his field, which my advisor and I are not familiar with, so the model might not work. I wasn't sure how to respond to him at that moment. Fortunately, the issue only associates with part of my talk. And, my advisor was there, too. He responded to him that this part of the work is still in progress so I wasn't too embarrassed.
Therefore, I would like to ask, in general,
How should I respond when an audience points out that there might be an issue in my work during my talk?
How often does this happen in an actual talk?
How serious is this?