1. Toxic team members who can't be fired. These people will reduce morale down to zero and force your best people to leave. In one case it was the CEOs long-time friend, and in the other case it was one of the technical founders. If you find yourself as an employee in a similar situation, then go ahead and pack things up because it will never get better.
2. Selling to Fortune 500s without investing in a sales team. Basically, we built it and they never came because B2B at this level is much much different than B2C. So if this is your move then your sales team should be bigger than your engineering team by at least 2x.
3. Toxic team members in recruiting positions. I once worked with a business partner whose recruiting strategy boiled down to bullying people into joining the company. After a year's time we had a solid 12 person engineering team while the business team was exactly the exact same as when we started.
4. Focusing too much on writing code and not enough on building relationships in your domain. When it comes time to sell, you will need to reach out to someone, so start nurturing relationships early.