Planning for your retirement is tough, but for couples, there are even more obstacles to putting together a smart financial plan. Yet despite all their efforts, many couples end up making a simple but devastating mistake in retirement that can cause huge problems down the road.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, looks at this simple mistake: not having both members of a couple know about the couple's finances. Dan notes that in many cases, only one person knows about the couple's finances, managing investments and paying bills. That can leave the other person completely unable to manage the couple's finances if something unexpected happens. Dan suggests that the ideal situation is one in which both members of a couple participate equally in financial decisions. But he concedes that that won't always happen, and so he also suggests that the person who takes responsibility for finances at least give detailed instructions on how to find financial accounts, what bills have to be paid, and who the other person can turn to for help. By addressing this simple mistake, you can make sure you don't leave loved ones in a bind if something happens to you.

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Dan Caplinger and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.