A 401(k) loan is a convenient way to borrow money in a pinch: You don't need a credit check, you can get the money pretty quickly, and you pay it back to yourself with interest over time.

But before you agree to one, make sure you understand the fine print. There's one little-known rule in particular that could leave you worse off than you were when you first took out the loan if you don't know it's there.

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Generally, you have five years to pay back a 401(k) loan. If you fail to pay it back as scheduled, the IRS considers the outstanding amount a distribution, and you'll pay ordinary income taxes on it, plus a 10% early withdrawal penalty if you're under 59 1/2.

You should know what your payments will be when you take out the loan. But if you haven't read the fine print, you may not realize what can happen if you quit your job before you've finished paying back the balance. In some cases, you might be asked to repay the entire outstanding amount at once.

If you're not able to, you could face a huge tax bill this year, and you'll hurt your 401(k)'s growth. So a 401(k) loan may not be your best option if you don't expect to remain with your employer for long.

You might consider another option, like a personal loan, that lets you use the money for just about anything. These loans tend to have higher interest rates, and of course, you'll be paying a creditor instead of yourself. But you'll be able to leave your retirement savings untouched so they can continue growing until you're ready to leave the workforce.