Probably a sure fire way to ruin your future.
You are slowing down the growth of your retirement fund. The money you withdraw stops growing until you pay it back. Some plans don't allow you to make more contributions if you have an outstanding loan, which hurts your retirement savings even more.
You repay the loan through payroll deduction, so the loan will cut down your take-home pay.
If you leave your job either voluntarily or involuntarily, you have to repay the entire outstanding balance in 60 days or face a huge tax bill. This can be very difficult to do when you are leaving your job, particularly if you are laid off. If you don't pay it back, the remaining balance is hit with a 10% early withdrawal penalty and you have to pay federal and state income taxes on it that year. So if you had borrowed $50,000 and couldn't pay it back, you would have to pay a $5,000 penalty and federal and state taxes that could take another $20,000 of the amount.