I'll answer based on your response to my comment on the OP.
The basic issue is that when income from multiple jobs is combined, the marginal tax rate may be higher than that for either job separately.
For a single taxpayer with multiple jobs, this is easy to figure out. Suppose you have a job where your AGI puts you at the very top of the 15% bracket, so that if you earn one more dollar there, it will be taxed at the 25% rate, and your employer will know that and withhold accordingly. Now suppose you take a weekend job that will earn you $9000 during the year. Even if you fill out a W-4 with your new employer indicating no exemptions or allowances, they will only withhold at the 10% marginal rate ($900), while you should be paying 25% on that income ($2250). That will leave you with a $1350 tax bill when you file in April.
It's a little more complicated for married couples, but the idea is the same. If you mark that you are married on your W-4, even if you take no allowances or exemptions, the employer will use the "Married" withholding table, which assumes that the income from that job is the entire income for the couple. Since the marginal rate increases more slowly for married couples than for singles (in aggregate, not per person), taxes will be withheld at a rate that is too low. (You may also be subject to the marriage penalty, but that is a correction on the main point.) So, the incorrect withholding comes from the fact that neither employer knows about the other for the purposes of figuring the marginal tax rate (and thus the withholding).
For two-earner households (or multiple-job earners), there is a worksheet at the bottom of Form W-4 that will allow you to compute the additional withholding necessary to avoid an end-of-year tax bill. At minimum, you should mark your W-4s to withhold at the single rate.