Some fear that Congress will cap the value of the tax exemption for munis, as the Obama administration has proposed doing to raise revenue.

WSJ: Conquering Retirement: The Case for Muni Bonds

If the estate's executor misses that crucial filing, the couple will likely lose the value of one partner's estate-tax exemption.

WSJ: Tax Report: Avoiding Estate-Tax Traps

In past years, the gift-tax exemption was often much smaller than the estate-tax exemption, which limited the early transfer of assets that were growing in value.

WSJ: How Much Will Your Taxes Jump?

However, one lesson is that tax exemption is worth a lot, and that an organization that loses it loses something of enormous value.

FORBES: No Gays In Boy Scouts? No Tax Exemption Either

Raising that exemption level is aimed at softening Democratic criticism of the proposal because it means only people with high-value health benefits -- so-called Cadillac plans -- would have to pay the new tax.

CNN: Baucus: Lawmakers $320 billion short for health care plan