When you try and try, but don't succeed. Failure is an inevitable result of learning how to do something new, and therefore of encountering uncertainty and of attempting innovation. Failure and growth are inextricably bound together. Unfortunately, it is often the case that people don't even try at all—because they fear failure in irrational, emotional ways. Designing situations where failure is unavoidable (such as desperation by design and other forcing functions) can help get around this problem.
See also: ambient uncertainty, comfort, discomfort, goal-setting, negative capability, not-knowing, open-endedness, productive discomfort, routines, self-therapy, the work of uncertainty, uncertainty mindset, voluntary uncertainty, work-in-progress.