Success on a project has to be measured in many dimensions and according to changing circumstances. As Sergio Flores commented, there is risk inherent in every project. There are things beyond the control of the project manager and in some cases project managers simply make mistakes.
When a project manager takes on a project, he or she enters into a partnership with the sponsor. The sponsor depends on the project manager's ability to bring the project to fruition, and the project manager depends on the soundness of the sponsor's ideas. They share in each other's potential success and each must be willing to face the risk of failure.
It's human nature to inherently push ourselves beyond our limits. The willingness of a sponsor and project manager to enter into a partnership knowing that there are risks is a testimony to this spirit.
I believe that the very possibility of failure makes success all the more exhilarating. And because we're social creatures, I believe the possibility of mutual failure makes mutual success all the more exhilarating.
Is it a disservice to the sponsor for a project manager to enter into a partnership when there is a distinct possibility of failure? I think not. In fact, I think it would be a grave disservice to decline to do so for fear of failure.
At a personal level, I rather like the idea of my sponsor and I betting on each other to succeed. We could lose, sure. But the fact that we could lose, together, makes the possibility of winning together all the more compelling.
What do you think about mutual success and failure with your sponsor?