No-one is perfect. Everyone can make a mistake.
You can miss a typo in your thesis. You can mis-remember a reference in the viva. You can not-quite-catch the importance of a question.
Your examiners can not-quite-get a concept you write about. They could mis-hear you. They could not recognise a typo as a typo.
And they know you could be nervous. They could be nervous. Exams make a lot of people nervous. That doesn’t mean there is something wrong with you, your thesis or the viva.
Everyone is human. Remember that and you’ll realise that the little human imperfections don’t add up to much compared to the achievement of your research, your development as a person and your talent on the day.