Things might go a little wrong in your viva.
You might forget a word.
You might mis-pronounce something.
You might go blank.
You might mis-remember a reference.
You might start a sentence and half-way through lose your thread.
You might have no other response apart from “I don’t know.”
You might forget your examiner’s name!
You might forget your own name!!!
All of these – at their absolute worst – are fumbles. They’re what happens occasionally to the most highly-talented, knowledgeable and capable people in moments of pressure. They keep you from being perfect in that moment.
And they’re not awful or disqualifying. They happen in lots of ways and for lots of reasons, and if they happen to you in your viva you have to breathe, do what you can and move on to the next thing.
You won’t fail for a fumble.