The vivas that take place over Skype.
The third examiner in the room, or the second external.
The examiner that isn’t an academic.
These aren’t what most candidates would expect, but they don’t make your viva unique and unpredictable. Your viva is unique because you’re unique, your thesis is unique – no-one will ever have the exam that you do. If you have a viva over videoconferencing, or if your examiner doesn’t have a PhD, you shouldn’t worry. These things don’t happen all the time, but they aren’t happening for the first time in your viva either.
They’re not common but the edge cases still have expectations. You can ask and find out about them.