[physicsguy]

That's really not the case here in the UK, I know several people who failed their vivas and had to make substantial changes to their thesis. It's not common, because there are intermediate level mini-theses and vivas, and people who do badly in those often don't finish at all, but it does happen.

[conjectures]

Yes, in UK mine was terrifying. In all likelihood candidate would get destroyed trying that tactic.

[nicioan]

I second that statement. I know at least two people who failed their viva in the UK and ended up getting an MPhil instead of a PhD. In both cases the candidates were told by their advisor not to go with the defense but they decided to go for it anyway. I know several more that got “major corrections„ which meant a lot of extra work and another review six months later. edit: typo

[shatnersbassoon]

My supervisor had a student who apparently just said "no" as her answer to each critique, 38 times. She got the level of fail which is "never ever come back" - which apparently does exist.