Why is it that lessons learned the hard way are often the ones that stay with us the longest? It's so much easier learning from the mistakes of another. Things such as "never start your business two weeks after the Yellow pages ads need to be submitted when your business relies upon that advertsing as a primary way of gaining new customers."
This list is priceless: Michael McDonough's Top Ten Things They Never Taught Me in Design School. It's scope goes way beyond designing.