The only time you don’t want to fail is the last time you try a thing.

Charles F. Kettering (1876-1958) American inventor, engineer, researcher, businessman

“Don’t Be Afraid to Stumble,” The Rotarian (Jan 1952)
    (Source)

This was a favorite phrase of Kettering's, and many versions exist or are attributed.

• "The only time you mustn't fail is the last time you try." (The Rotarian (May 1953))
• "The only time you don't want to fail is the last time you try."
• "The only time you can't afford to fail is the last time you try."
• "The only time you don't fail is the last time you try something, and it works."