In the viva, no candidate wants to say the wrong thing. No-one wants to misremember a detail or misquote a paper. No-one wants to go blank and say the first thing, the silly thing, the wrong thing.
No-one wants to say the wrong thing, but remember there might not be a right thing.
Not every question has an answer. Not every question is probing for truth. A question could be exploratory. A question could be to clarify a point. A question could be seeking an opinion if there is one.
In the viva you could definitely say the wrong thing; depending on the question you might not be able to offer a right thing. You can always take your time and offer your best. Listen to the question, pause, think and respond.