What is it that you wish you knew or understood before you started your project?

There are lots of unknown unknowns when you only have the idea. Known unknowns can help you starting but factors you're unaware of initially can bring your whole idea down or make things 10x more challenging. Do you have anything you wish you knew before you started your project? I'm not a founder on our team but here's mine as the marketer / developer relations coordinator of our team: I wish we knew that open-source is a necessity for our product. I don't think we're late with anything, considering how our product isn't fully ready yet for even public beta, but we would've saved many unnecessary laps with our communication. Another, rather personal thing I wish I knew is to just start engaging with our target audience. Being a non-technical person working on the marketing of a product that solves a very technical problem I was afraid of being exposed as a dummy. I just let it go and continuously learn from others while talking to them. What's yours?