Do everything yourself! Know where you are the best and hire out the rest!
Neglecting your health! Running a business is demanding physically and mentally. Make time for regular exercise and eat well. It pays.
Skimping on marketing! Well I couldn't leave this one off such a short list considering it is what I see every day. Other problems seem far less serious when new customers are coming in the door regularly. Marketing is the lifeblood of your business. Do it or sink it.
Kill or be killed.. aka "Unethical practices". Stealing, cheating, price gouging, scamming and the like may not only land you in jail, destroy all relationships, and ruin your reputation, but will also destory any chances of making a good living in the business world. Best word on this comes from the book of Numbers "Be sure your sin will find you out"
Outgrowing your britches before you buy new ones. In other words, growing to fast. Growth takes proper planning. Whether htat mean buying new britches, making sure you are staffed up with the right talent in the right areas, or getting a loan to deal with cash flow issues.