This issue most commonly arises when there are unpaid taxes from joint filing years, and a couple later separates or divorces. The IRS can pursue either spouse for the full amount. If you're the easiest one to find, or if you have liquid assets, you can end up paying the entire bill.
When this happens, the only relief is the so-called innocent spouse rule. If you can prove that you had no reason to suspect tax shortfalls and you did not personally benefit from unreported income, or that you signed joint returns only under duress, you may get off the hook. Unfortunately, this is easier said than done. The IRS and the courts have been notoriously stingy in allowing innocent spouse relief.
What can you do to head off trouble? First, consider the obvious. If your family spends much more money than the income shown on your tax returns, warning lights should go on. If you don't understand all the tax and financial issues in the joint return, ask questions. In certain circumstances, you may even want to consider hiring your own tax professional to advise you before signing.
If you are headed toward separation or divorce, it may be best to file separately. You may pay a little more tax, but that's better than leaving yourself liable for the tax sins of someone who is no longer on your side. Don't file jointly unless you're sure that all income has been reported on the return and that the taxes have actually been paid.