A question you have not considered. A question that surprises you. A question that does not seem relevant.
These questions could be scary: if the goal of the viva is to engage in discussion then a question you have not considered could be terrifying.
But an unknown question does not mean the answer is unknown. A question you’ve not thought of before can still have a response.
You got to where you are by answering questions you didn’t know at some point.
You know a lot, you can do a lot. Answering questions is part of your skill set.