Mixing Family and Business Relationships
This is simultaneously the most obvious pitfall and the most difficult to avoid. And if you've started a family business, it's already happened. As soon as personal relationships get strained, you're risking the business -- try running a shareholder meeting when no one on the board of directors wants to talk to you.
Priorities have to be set early so that you know what's going to be sacrificed when business life and personal life butt heads. Or vice versa. It may very well be that you'll have to destroy one to save the other, so know in advance which is more important in case you have to cowboy up and put half your life on the chopping block.