Have you ever had an unsuccessful project and what did you learn?

I can't recall any project that was unsuccessful, but plenty that I have learnt a lot from and wished I could go back in time to do a few things differently.
A lot of what I've learnt comes naturally with experience - when you are working with technology there are sometimes bumps along the way because you don't know what you don't know - so always ask for help if you need it, or double-check that your understanding is correct and complete.
When working with customers (people) it's important to manage expectations, don't over-promise and under deliver. Set realistic timeframes for actions to be completed and deliver early where possible.
I'm a detail-oriented person so in the past I've lead conversations down a rabbit hole only to find that we're going around in circles which leads to everyone feeling frustrated. A big lesson I've learnt is to focus on the outcome, listen to what the customer is looking to achieve and provide the best advice I can, rather than overwhelming them with technical options.