[Entrepreneurs] make lots of mistakes. They overestimate market size and underestimate how long things will take. But what happens most often is that entrepreneurs underestimate the importance of what they don’t know. They know their competition as it is today, and they know that their own product is going to be significantly better. But they don’t know what’s going on in the backroom of their competition. You can’t discount the possibility that the other guy is getting ready to replace the Model T with the Model A.