What's a failure that ended up leading to a new opportunity or success?

Not really a failure, but my project started after paying around $500/mo for billing software that still didn't do everything I wanted it to, so I started my open source project and built the things I needed! And while the project not actively making me money, the success is that my business now runs more efficiently and less mistakes have been made during the quoting and invoicing process.. So, still a win!