What mistakes should people make at least once in their lives?

I think you let people work with you then they back out. From "back out" you can see how people treat you. What they need and what they want to focus on. From there you can understand more. But some things are unfortunate.

I would say: working on the wrong thing. What I mean by that is either working a job you don't like, or working on a product you don't like but has a good opportunity. We went through the latter. You need to experience some bad to truly appreciate the good. For us, working on something we were passionate about made it a lot easier to push ourselves and go the extra mile.

Maybe diving in a project when being unprepared. Gets you to realize what is important and what you can spend less time on. But it's quite stressful as well so I also don't know if this really is a good mistake to make intentionally haha

Believe in idea and build product without custdev :)))

Take an unplanned journey

I think by going for the things you are not much sure, what will be the output but you wanted to try. Many successful people have tried these and that is what that made them successful.

On the lighter side, forget wallet when you go on a holiday with friends. Oops..

Have a company fail. I've learned a TON in my time of fighting to earn what what one day I would want to keep. Those times building a "wartime CEO" mentality are invaluable.