Using your 401k like an ATM. I've been guilty of this myself. There is no other (legal) way to get money so easily. With my plan, I can log in and go to the loan section and it will give me a preapproved loan amount and the interest rate. If I choose to borrow the money, I can have the money in my checking account in as little as 48-72 hours. By borrowing money from my 401k, I am losing the opportunity to have that money grow and compound in my account. Not only that, if you leave the company, your are required to pay the money back immediately or be subject to taxes on the money and pay a 10% penalty.
Too much company stock. It is certainly not a good idea to have too much company stock in your retirement account. If the company goes under, not only would do you lose your paycheck, you will lose your retirement money too. Remember ENRON!
Not fully understanding your options. One of my coworkers has been with the company for 5 years. She did not understand how the 401k worked and only elected to contribute 1% of her pay. Not only that, she directed 100% of the money to a money market type fund. I actually spent a few hours after work explaining her 401k options and she's made some major changes. She is now contributing 5% in order to get the full company match and she has chosen a more diversified portfolio.