Failure stories teach people better lessons than successful ones. Can you share one of yours?

I think a simple format like this would work best. 1- What was the idea 2- How much time did you spend on it 3- Why do you think it failed I'll start first: The idea was to build an international online ordering and delivery network for Birthday Cakes, just like the Flower networks. I spent about 9 months, almost full time. Because I had a long experience with flower networks, I was overconfident that I knew the market and operational ins and outs. What I found out, was that the majority of bakeries, in contrast to flower shops, do not have an established delivery procedure, and standardizing it across the board, turned out to be impossible. I should have better validated the idea, and be more careful with my assumptions.