There are many things that can go wrong when you create a project and turn it into a startup so let's not generalise too quickly. However one big driver to keep in mind is the market.
|Lessons learnt: never loose eye contact with your target market!|
Disconnect from your target market and you're sentenced to death. Yes you need money, a good team, a business plan... But even if all goes wrong you can still rely on what you've learnt of the market to raise money, gather a team, build a new plan.
That's why it often comes down to the target market. Get to know your market as early as possible and you're safe. Don't hide, show some of what you're working on, learn all about the market.