Why might I fail? What happens if I fail? What is the process for failing the viva like?
I am asked questions like this all the time. Some people do fail the viva. The vast majority don’t.
What makes you think you would?
If you’re thinking about failure, seriously thinking about failure… Why? What has tripped that thought for you? There’s a real difference between wanting to pass and thinking you might fail. If you’re concerned about your examiners, read some of their papers, ask around about them. If you’re worried about remembering “everything” then read your thesis carefully and make some notes. If you worry about answering questions under pressure then have a mock viva, or get friends to practise with you.
If you’re really troubled by the thought of failure, do something about it.