Here are my top 3:
1. Wasting money on a legal entity. If you're not making money, you cannot be sued. Wait until you make money or have someone willing to give you money before you waste money on protecting yourself.
2. Bad business partners. Judge your partners not by how much you like them but if they'll complement your skillz, add value to the team, and are dedicated to the project.
3. Not saying no. It's a personal problem, I'm a really nice guy, it's hard for me to say no sometimes, but it's necessary. It's not personal, it's business.