Recently there was a discussion on an internal mailing list about when a sprint was was considered a failure. Specifically the question was asked if seven out of nine user stories completed meant that it was a failure. Naturally that should be considered a failure! NOT! As far as I'm concerned there is no such thing as failure in Scrum. Scrum is not a method that can be used to measure success versus failure. Scrum is a method that helps you understand what your real problems are. As long as you learn something and continue to improve you cannot fail. If you ignore all signals and refuse to learn from your mistakes (and successes) then you fail.
One thing that probably is the source of confusion is the sprint reset. That is before the end of a sprint the team (or product owner) realizes that the current plan is just wrong and that the best thing to do is to stop the running sprint and plan a new sprint instead. While this should be considered an exceptional case and if it happens a lot you need to understand why. But it should not be considered a failure, it's a great opportunity to learn.