One of the biggest mistakes happens when you get desperate for money and take out a 401k loan. If you continue to take out these loans, you will not have much of a retirement. When money is taken out of a 401k for a loan, it is not earning money. Over a 20- or 30-year period, there are only a few months when an investment has enough growth to make it worthwhile. Another problem arises if you don’t pay the loan. The IRS will smack you with huge taxes if that happens. If your money is tied up in a 401k loan when the stock market is hot, you will lose out.