One mistake is to vastly over-estimate the amount of capital needed to start an enterprise, and to simultaneously vastly overestimate the anticipated revenues (cash flow).
A big mistake is not to START, ASAP. Generally it is possible to start any enterprise on a shoestring, and to expand using profits generated. It is also easier to get angel or other investors or loans when the project is already an existing reality with actual customers and not just an “idea.”
Another mistake is to waste time and effort into a project or enterprise that has no chance of success. I always tell my proteges, those who I am mentoring, that they need “adult supervision” because some of the ideas I hear are for hairbrained projects producing products or services that nobody wants or will buy.
In my Think Like A Tycoon book, I have a chapter about my experiences with inventors: They are usually totally ignorant of the need for such things as marketing, quality control, test marketing, and the “real world” in general.
My 65 years of business experience and a few dozen jobs and many business deals have taught me a few things: There are such things as profitable niche markets, but usually it is possible to do a marketing survey asking, “Would you buy this product at this price if it were available next month? Would you be willing to order it now?” If there is absolutely no interest, it is time to move on to another project.
But some entrepreneures get stuck on a dream that has zero chance of making any return on time and money invested.