"You can't disinherit a wife from 401(k) funds w/out spousal waiver."

Your will: The hidden traps

By Janice Revell, contributor @FortuneMagazine August 22, 2011: 1:30 PM ET

FORTUNE -- Drawing up a will may not be the most pleasant task, but it seems straightforward: You leave behind a legal document that specifies how you want your property doled out when your time on this earth is up. What you may not realize, though, is that much of your net worth can be passed along outside of that will. Without some planning, you could unwittingly disinherit intended beneficiaries, including your children, from significant portions of your estate.
Start with your 401(k) plan. If you're married, your spouse is automatically entitled to every dime in the account when you die, regardless of what your will or the beneficiary form says. And that applies even to accounts you established with former employers years before you met your better half. If you want to leave a 401(k) to someone else, your spouse must first file a written statement waiving rights to it. In rare situations your 401(k) plan may not require this spousal consent if your marriage is less than a year old. But that's not the norm, notes Ary Rosenbaum, a retirement-plan lawyer in Garden City, N.Y. In most cases, your spouse will become the sole heir to all your 401(k) accounts the minute you say "I do."

Don't count on a prenuptial agreement to solve this kind of quagmire either. A person can't give up spousal rights to inherit a 401(k) until actually married. "A prenup by itself is not a valid waiver according to the rules governing 401(k) plans," says Rosenbaum.

Then there's the money sitting in your IRA accounts. In most states your will has no bearing on who inherits any of your IRAs at the time of your death. The person who gets the cash will be the one you named on the beneficiary form. And it doesn't matter how long ago you named the recipient. A spouse you divorced 30 years ago, for instance, will usually collect if his or her name is still on the form.

Here, at least, you've got a bit more flexibility. In most states an IRA, unlike a 401(k), doesn't revert automatically to your current spouse when you die; you can name anyone you want as beneficiary. Beware, though: Once you're married you can't transfer the assets of a 401(k) account into an IRA and then name a new beneficiary -- effectively disinheriting your spouse from the 401(k) -- without obtaining your spouse's written consent first, notes Ed Slott, a CPA and IRA specialist in Rockville Centre, N.Y. But if you're about to get married, it's perfectly legal to roll a 401(k) account into an IRA before you walk down the aisle.

Finally, remember that another financial asset not governed by your will is life insurance. Once again, those who get the money are the people you named on the beneficiary forms. The bottom line: It's crucial that you get the full picture of who really stands to cash in on your estate. Review the beneficiary designations on your retirement and insurance accounts on a regular basis, and make sure they're in sync with the intentions outlined in your will. After all, it's your loved ones you ultimately want to enrich at the end of the day, not their attorneys.

--A former compensation consultant, Janice Revell has been writing about personal finance since 2000.

This article is from the August 15, 2011 issue of Fortune.