I have heard of cases of people who have failed but don't know of anyone personally. I would like to think it is relatively rare as it hardly reflects well on the supervisor etc.
I had these worries myself but I think the mock viva helped as well as this forum! I would recommend reading the Tinkler book that someone else suggested on another thread. I also went to a Uni session about the mock viva and that was really useful. They outlined the possible options (e.g. pass no corrections, minor, major, mphil etc etc).