The No. 1 mistake most people make with investing accounts—you can fix it in 5 minutes

When it comes to investing accounts, such as 401(k)s and IRAs, financial pros will generally tell you, the less you look at them, the better.

As long as you have a broadly diversified portfolio that matches your tolerance for risk, the thinking goes, not much good can come from worrying about the daily ups and downs of your investments. Instead, sit back, and watch compound interest work its magic over the decades you're invested.

There is one major exception to the rule, however, and it has nothing to do with the stocks, exchange-traded funds or mutual funds you own. If you aren't regularly checking in to make sure your beneficiaries are up to date, you could be making a costly error, says Ed Slott, a certified public accountant and founder of IRAHelp.com.

"It's the no. 1 mistake most people make," he says. "They think it's all taken care of. But the beneficiary form overrides the will. Most people don't know that. And they don't update their beneficiary forms."

Here's why, if you haven't updated your beneficiaries since you opened your accounts, it's worth checking in ASAP.

In the event of your death, your brokerage or plan administrator distributes your assets to whomever you have designated as your beneficiary. Failing to name a beneficiary at all can result in your loved ones having to duke it out in court in an often lengthy and arduous process known as probate over who receives the money.

To name a beneficiary, you'll have to log on to your brokerage or plan administrator's website and enter some information about who you want to receive your assets, usually their name, date of birth and, optionally, Social Security number.

This process generally only takes a couple of minutes, and you likely breezed through it when you opened your accounts. So why check back in? Because your situation may change. And if you die before changing your beneficiary designations, the wrong people may get your savings.

Slott cites the recent case of Jeffrey Rolison, who enrolled in the Procter & Gamble 401(k) plan in 1987, naming then girlfriend Margret Losinger (nee Sjostedt) as his beneficiary. The couple broke up in 1989.

"Over the years, they sent reminders to him. 'We're going digital — we're moving beneficiary forms online. Do you want to make a change?'" Slott says. "Over all those years, he never changed it."

So when Rolison died in 2015, a woman he broke up with decades before received about $750,000 he'd saved in his account. His estate contested the inheritance, and lost.

To avoid a similar fate for your loved ones, Slott recommends updating the beneficiaries on all your accounts at least once per year. "You should also have your antennas up when there's what I call a life event," he says. "A birth or death. A marriage or a divorce. A new grandchild, a change in the tax law. These are things that should be looked at."

Beneficiary designations typically supersede a will or trust and are often paid out directly by your brokerage or plan provider to the person or people you designate. And make sure you find a way to give the people you name a heads up. In the case of Fidelity, for instance, "beneficiaries must contact Fidelity themselves to receive their assets," company policy reads. "We'll distribute your assets to your beneficiaries without requiring a will or other legal documents.

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