New York Times columnist Paul Krugman famously dubbed the Bush 2001 tax cuts the “Throw Momma From The Train Act”, because the estate tax was eliminated for just one year—2010. But as 2010 grinds on without a federal estate levy, it’s becoming clear that Krugman got it wrong. Any Momma who would ride the rails (even the pricey Acela) probably isn’t worth shoving to a grisly demise. It’s the Mommas flying on their private jets who need to pack parachutes or watch their backs. Without a doubt, the one- year lapse in the federal estate is a boon to heirs of the superrich. (At least four billionaires, including George Steinbrenner have died so far this year.) But for ordinary families, it is creating all sorts of grief and unintended consequences and might even cost some of them extra federal tax, to say nothing of lawyers’ bills.

One set of problems relates to wills that were written assuming there would be a tax; provisions in such documents could inadvertently disinherit children or a spouse, or could subject an estate to unnecessary state estate tax. (For more on these issues, click here. For a map showing 2010 state estate taxes, click here.)

Another set of problems relates to a trade-off Congress made in the 2001 law: In return for eliminating the estate tax in 2010, it also did away with the full “step-up” in basis for capital assets for 2010. In other years, the basis cost of a decedent’s capital assets–stocks, bonds, jewelry, real estate, artwork and so on– gets adjusted to its market value at his death, or six months afterward. (The executor gets a choice.) Conveniently, that allows heirs to sell all the property immediately with no capital gains income taxes due. But for those dying in 2010, the step-up in assets going to non-spousal heirs is limited to $1.3 million, with another $3 million in step-up allowed for assets left to a spouse. This means some heirs of estates worth several million could end up paying more in total federal tax than they would have had their benefactor died in 2009, when all assets got a step-up in basis and the first $3.5 million of an estate going to non-spousal heirs was exempt from estate tax. (Amounts left to a citizen-spouse aren’t subject to estate tax, since Uncle Sam figures to get his when the second spouse dies.) These moderately well-to-do families get hit with extra capital gains taxes because their benefactor died in 2010 instead of 2009, but they don’t save much or any estate tax, compared to 2009

While an unknown number of families may pay more, a greater number of them are having to shoulder a big paperwork and administrative burden. Assuming capital assets (including a home and stocks) total more than $1.3 million, family members and executors must locate old records showing what assets were purchased for (if such records even exist) and deal with all sorts of complicated and potentially divisive issues such as which assets, going to which heirs, get allocated the limited basis step-ups…

Considering the complicated nuances, it might be wise to keep Momma – and her private jet – around for another year.