To err is human, but some mistakes are worse than others, and slip-ups that occur while you're planning for retirement can come back to haunt you financially.

But it may not be too late for you to fix some common mistakes. Here are five prime examples:

1. Saving too little. It seems obvious, but not setting aside enough money could become a big problem if you underestimate the amount you'll need to live on—all the more likely as life expectancies continue to rise. So if your employer offers a 401(k) plan with matching contributions, try to take full advantage of it, even though your take-home pay will be reduced by deferrals. And you can supplement these savings with IRA contributions.

2. Starting too late. From the start of your career there are many financial priorities competing for a share of your salary. You may be saving to buy a home or to put your kids through school. Yet while early contributions to a retirement plan can produce outsized benefits, you may be able to make up for lost time if you put as much as the law allows into your retirement savings. For 2017, the maximum 401(k) deferral is $18,000 or $24,000 if you're age 50 or over. The IRA limit is $5,500 or $6,500 if age 50 or over. You also might decide to work a few years longer than you'd originally planned. That can boost your savings while reducing the length of your retirement.

3. Ignoring taxes. Taxes are an essential part of the retirement planning equation. When you take money out of your retirement plans you'll likely owe federal and state income tax on those distributions. Part of your Social Security benefits also is subject to taxation. And your tax rate during retirement might be higher than you expect if you don't get some of the deductions you were able to claim while you were working. Factoring in taxes when you plan for retirement will help you create a more realistic scenario.

4. Not diversifying your investments. While you've undoubtedly heard about the benefits of spreading your investment dollars across many kinds of holdings, it's often tempting to stick with investments that have been doing well for you. But there's no guarantee that gains on a particular stock or fund will continue, and creating a diversified portfolio can help reduce the risk that you'll be hurt by losses in one or two investments. Just keep in mind that diversification doesn't provide guaranteed protection, especially in declining markets.

5. Ending retirement planning when you retire. Even after you retire you'll have important decisions to make. You'll need to make sure your portfolio stays diversified, and you'll likely need to allocate some money to stocks or other investments that may help you keep pace with rising costs.

Maybe the biggest overall mistake you can make is assuming you know it all. Reach out for expert assistance to avoid the common traps.