hi, what do you mean by viva? is it a defense! here in North America we have comprhensive exam after about 2 years of your ph.d program. then if you fail it you got only one single chance to do repeat it. if you fail that you are out of your ph.d. and I heared few instant about people being kicked out completely from the ph.d because of that, but it is rare. no for defense I did not hear about any one failed that. I think supervisor should not allow the student to go throughe defense if they are not sure!! scary

I have heard of quite a few people failing for a number of different reasons - first they have reached the end of their uni enrolment period and are forced to submit no matter what; second they are very mature students treating it like a hobby rather than the professional qualification it is (in which case the first is a possibility); third they have seriously plagarised; and fourth they just didn't have what it takes and their supervisor let them submit all the same. These comments are not meant to offend merely give some examples that I have heard. No personal injury intended.