If you break eggs to make an omelette, you never really expect any of them will be rotten. There are processes in place that mean a box of six free range eggs on a supermarket shelf are as good as they can possibly be when you buy them. Treat them right and they’ll be fine when you need them.
A bad viva is like a bad egg: it’s a possibility, but it’s rare.
You shouldn’t expect your viva will be bad or you will fail. A good, successful viva isn’t a fluke, it’s the norm.
Expect to pass and work towards that outcome.