The most common mistake is working on something that just isn't really a problem, or it's just not a problem that people care enough about. Here's an example of what a good problem looks like. Brakes from winter 2017 makes a credit card for startups. Before Brakes, if a startup NYC wanted a corporate credit card, they literally could not get one because a no bank would give a credit card to a startup. That's a good problem. And next, do you have competition? Most good startup ideas have competition. But if you are going up against especially entrenched competition, you typically need a new insight.