When failure is a good thing.

What is one thing that successful people do... they fail and fail often. This key is their attitude and what do they learn from that failure to make them a success. John C Maxwell's book "Some times you win Some times you learn" is an invaluable book to teach about the attitude of learning from our failures.
What are lessons you have learned from your failures? Have you ever purposely failed at to something to learn something important?