What are common mistakes of entrepreneurs you've witnessed over the last 40 years?

I've learned that success in entrepreneurship is often driven by overconfidence and being able to form a relationship with investors so that if tests are promising, they will give more money even if the plan wasn't achieved. Apple's success story demonstrates this, as VisiCalc created an unexpected market for business computers despite their initial plans. The importance of not running out of cash or trust when forming these relationships cannot be understated.
 
Transcript: "I've read, maybe, 10,000 business plans. I don't really know at this point. But I've only seen three companies actually meet their plan. And I think that's a pattern. I don't know about other people but-- so you're almost always overconfident. And without overconfidence, there's no parenting, there's no entrepreneurship. So I think what I've learned is you have to be able to form a relationship with your investors such that, if your tests are promising the things you've learned about customer product-market fit, then they will give you more money, even though you didn't accomplish what you said you were going to accomplish in your first slide deck or business plan. Apple missed by 60%-70% in the first year. Apple didn't think there was going to be a market for business computers and then somebody invents VisiCalc. The market for business personal computers explodes. So again, they could have said, you didn't do what you were promising. We hate you. We're going to take away the money. No, I've got faith that you're going to be able to figure this out. Don't run out of cash. Don't run out of trust."