"I think it's more common than most people realize," Hoyt says.

While trusts are hugely popular and are great estate-planning tools, they're not foolproof, Hoyt says.

"Unlike wills, they require no court supervision. So where there's no formal court supervision, there can be a lot of abuse," she says.

As in a divorce, the battles that ensue when an estate plan is challenged can whittle an inheritance away to nothing, as the family is forced to pay out hundreds of thousands of dollars in legal fees. To avoid that, estate planners say clients should be very specific about where they want their assets to travel after they die. And if possible, place those assets there themselves, so there's no question about their intent.

"Sharing the estate plan with all of the beneficiaries is something I always advise. You will find very quickly if they approve or disapprove, and you can make the adjustments," says Dan Walker, president of Farmers and Merchants Bank in Long Beach, Calif.

With a second marriage, he says, there's almost always a party who disapproves. Even if everyone used to get along, the children can turn against the stepparent when their real parent passes away, Walker says.

Some stepparents deserve that animosity because they have a sense of entitlement, Walker says. They are far more interested in their own well-being and retirement funds than with protecting the children of their spouse, he says.

"When they start acting like that, it immediately causes a battle for the assets," Walker says. "You see the same thing among siblings, when the parents die and one of three children feels they are entitled to more than the others."

That's why it's vitally important to have a clear estate plan delineated. Walker knew of one woman who was on her third marriage, and within a week of her husband's death, all of his assets flowed into her checking account. There was no estate plan and nothing the children could do, and the wife saw no reason to remedy the situation.

"From a financial standpoint, she did very well," Walker says.