WHAT are the possible outcomes of a viva? Fail with no right of resubmission – this result is very rare and usually only happens if you have plagarised your thesis. Award of an MPhil – if the examiners feel that the amount of work you've done is not sufficient for a doctoral degree they may award you an MPhil.

• do not ask about special cases, e.g. supervisor does not approve the thesis, there is rivalry between the supervisor and the examiners and so on. Is there any general standard for viva, e.g. one needs to be able to answer 2/3 of the questions, etc.
• would like to emphasize again that I do not ask about special cases, e.g. plagiarism, wrong methodology etc. I believe theses cases are extremely rare. I just want to ask about a normal case where one managed to publish some papers, and advisor (happily) approved the thesis.
• his depends a lot on the culture, country, school etc. For instance if I want to do my state exams, I have to pass all previous 5 doctoral exams, and also pass 3 additional ones on the spot. Failing to say anything there would cause me to fail the exam quite for sure. 
• As a former PhD student, now lecturer, it's very rare to hear of one failing a PhD viva. The most common outcome is to grant the PhD subject to minor corrections, which will be checked off by the internal examiner. Occasionally candidates will have to make major corrections for review by the external examiner, and much more rarely, candidates will be told to revise and resubmit where the process may or may not include an additional oral examination. Only the last outcome would be generally regarded as "failure".
• In the first instance the student's advisor would not recommend/allow (depending on the institution) the candidate to go forward for a viva unless the work was to the standard required. In many universities, this means publishable in whole or in part, and making a non-trivial novel contribution. One way to satisfy yourself about these criteria is whether you've published anything at all to date: if you have, it answers the question that the thesis is publishable at least in part. It's very hard to "fail" if these basic criteria are met.
• About related approaches, you would probably be expected to have a rationale for choosing your own approach. This would imply that you know your chosen method well, and know enough about the others to be able to make a comparative choice. E.g. if you were working in electromagnetics and chose the Finite Element Method to solve a Partial Differential Equation, you would probably want to be able to point out why you rejected Finite Differences and/or Analytical methods. But I would not expect you to be able to discuss the intricacies of Finite Differences in any great detail. The working knowledge you already have of your own approach and that of others is probably quite sufficient, as long as you can provide a strong justification for why you chose the methods you did. It's quite OK to justify based on ease of use, local availability, expediency etc.

The outcome of your thesis will determine whether or not you pass your PhD viva. A thesis can be marked as a pass, in need of corrections, resubmission, downgrade or fail. Though you should aim for a pass, most students will be required to make some form of corrections.