If I knew I could not fail I'd get bored really fast. Half the fun in my life has been wondering of I would fail. It's a massive incentive to try your hardest and put your heart and soul into everything you do.
For instance, half the fun of asking out my girlfriend was the process of coming up with ways to ask her on dates and facilitate any feelings she may have had for me. If I had known that there was no chance of my failing, it wouldn't have been as much fun. Uncertainty is certainly one of the greatest pleasures. I truly prefer to wake up everyday wondering if my efforts will be good enough. If not, I enjoy improving myself. If so, then I enjoy celebrating my efforts and my successes. If there was no failure, victory would have no satisfaction.
So to answer your question, if I knew I would not fail, then I would attempt my hardest to fail.