Quotations about:
    fail

[T]he more education a man has, the less likely he is to invent new things. Possibly this is because from the moment the boy or girl starts in school he or she is examined three or four times a year and a failure or two and he or she is out. Now because an inventor works differently, he thinks that’s all wrong. He knows he’ll never go far on any problem before he strikes snags. He may flunk 999 times but if on his 1,000th try he succeeds, he wins! 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

Essay (1952-01), “Don’t Be Afraid to Stumble,” The Rotarian, Vol. 80, No. 1
    (Source)

Kettering, who was R&D Director at General Motors for many years, constantly emphasized the need for experimentation and, by definition, learning from experimental failures. He had a number of aphorisms and passages that were repeated by him on various speaking occasions, or quoted / paraphrased from him by others, particularly the last line above.

In T. A. Boyd's biography of Kettering, Professional Amateur, Part 3, ch. 20 (1957), we have:

It therefore seems that the only factor which needs to be corrected is to teach a highly educated person that it is not a disgrace to fail and he must analyze every failure to find its cause. We paraphrase this by saying, "You must learn how to fail intelligently." [...] For failing is one of the greatest arts in the world. [...] Once you've failed, analyze the problem and find out why, because each failure is one more step leading up to the cathedral of success. The only time you don't want to fail is the last time you try.

Here is this similar passage attributed to Kettering from a page blurb, "Don't Be Afraid to Stumble," Supervisory Management magazine, Vol. 2, No. 7 (1957-06):

We need to teach the intelligent person that it is not a disgrace to fail and that he must analyze every failure to find its cause. He must learn how to fail intelligently, for failing is one of the greatest arts in the world. Once you've failed, analyze the problem and find out why, because each failure is one more step leading to success. The only time you don't want to fail is the last time you try.

The shorter the phrase, the more likely it is to be quoted on its own, e.g.:

The only time you don't want to fail is the last time you try.
 

Which can be found in:

• T. A. Boyd, Research, ch. 22 "Persistance" (1935).
• Society of Automotive Engineers (SAE) Journal, Vol 42, No. 2 (1938-02), covering the Detroit SAE Annual Meeting (1938-01-10 to 14).

Other variants that can be found:

• "The only time you mustn't 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."