1. Don't bother creating any kind of cash flow model, because it's bullshit, right?
2. Overestimate the value and volume of the "data" that you'll collect when lots of users are on your platform.
3. Assume that lots of users will be using your platform when you don't even have a good plan to get the first 500.
4. Skip talking to the specific person at that big customer who you believe will one day spend lots of money on your platform.
5. Copy the strategies of Facebook, Twitter, Snapchat, Instagram, Uber, etc., because you fail to realize that those companies were developed in completely different competitive environments and also raised hundreds of millions, even billions of dollars in VC money to get where they're going.
6. Think of yourself as the "Uber for" when you don't even understand what makes Uber "Uber"--namely a commoditized, interchangeable workforce performing singular tasks whose demand is unpredictable geographically and schedulewise, that had lots of unused inventory.
7. Fail to appreciate what product management is--because you think that all you need is a technical co-founder to build something that people love.
8. Fail to understand the relationship between continuing to hire salespeople and growing your revenues, and fail to figure out what the payback is on hiring an additional salesperson.
9. Thinking that PR alone is going to get you millions of users.
10. Worrying about dilution and not raising enough money early on, and then raising far too much money later once you're successful--so much that you can't get out from under it.