You can’t know exactly what questions your examiners will ask, but you can have a good idea of the topics they’ll want to talk about in the viva. You can’t have a response ready and waiting for every topic, but you can feel fairly confident in your preparations that you can engage with almost anything your examiners might want to ask.
There’s always a possibility that they ask something you’ve never considered. There’s a chance they may notice something you haven’t. An unexpected question could be asked that you, at first, don’t know how to handle. You just might not know what to think or say.
Whatever the unexpected question, however left field it is, you can still engage with it. Pause to consider it. Think about what it means. Respond as best you can. Ask your examiners questions to unpick what they mean. Be patient with yourself.
Pause. Think. Respond.