I’ve been to many PhD orals either as one of the examiners or as Pro-dean (that is chairing the committee) and once as an interested observer. Only the last was there any chance at all of failing (it was a truly terrible thesis and well deserved to fail). In math it is embarrassing because only the supervisor even might have read it, so there will be few if any questions, which always perplexes the Pro-dean. When I have chaired committees, necessarily in other departments, it is amazingly different. The examiners ave all read the thesis and want to show off how smart they are. The Pro-dean has to keep them on message and also keep it from going on for four hours. The system that Cap’n has described is just too different. For one thing, the external isn’t usually at the defense. It might be more interesting if it was just the internal and external. But it pretty much limits the choice of external. Suppose he was in Australia.
Once upon a time, I happened to visiting a university in … another country. They asked me to participate in a final exam. The dean himself chaired the committee. The student was awful (I guess another case that deserved to fail) and when the committee met afterwards, I said so. I got a lecture from the dean how I was imposing my stricter, North American, standards. Damn right I was and if you don’t want my standards, don’t ask me. Finally, they wore me down (the “decision” had to be unanimous).
In my own defense, 48 years ago, I know nobody, certainly not my advisor, had read it. He did know what it was about, but certainly hadn’t checked the rather ghastly computations (a fellow grad student had, though).