1) Expanding your company into related endeavors without growing slowly and learning from your mistakes. A good example is that a lot of construction companies dip their toes into development and based one early success, they dump their whole pot of money into it and get killed by a simple mistake. 

2) Allowing  rapid growth to hurt cash flow. * 

3) Not watching expenses during boom times. In order to survive a downturn, you need low expenses and money in the bank  - but letting expenses get away and not socking a bunch of money away, surviving a downturn becomes hard due to lack of money.

4) Having one huge customer that will jerk you around, or kill your company when they go elsewhere.

5) Doing business with jerks, but thinking you're covered because you have a contract. If you have to enforce a contract, the damage has been done and it's too late.  *

6) Getting divorced.

7) Letting your children run wild for twenty five years and then turning the business over them. 

8) Never learning to delegate, and then when things get too busy to not delegate, turning a big portion of your company over to the first blowhard that shows up. *

9) Working so hard that your health suffers, or  you die.

10) Having a clean retirement - you stop working at all. You get bored and you die within two years.

11) Being in a market where other companies are going out of business -  killing your profit. 

12) Having a 50/50 partnership and one of the partners goes nuts and the the partners sue each other into oblivion. 

* Indicates that this has happened to us.