Forget about your 2008 tax return for a minute. If you didn't file one for 2005, you might be missing out on some money from back then. But you're quickly running out of time to claim it.

Three years ago, more than 1.3 million individuals apparently decided they had better things to do than file their 2005 tax returns, even though they were due refunds. In total, almost $1.3 billion worth of refunds from that tax year is still sitting in the Internal Revenue Service account.

Taxpayers can still get their old refund checks, which the IRS says could amount to around $800 for some folks. But the claim, via a 2005 Form 1040, must be made by April 15. After that, the federally allowed three-year window of opportunity from the original filing deadline, which was April 2006, closes forever and Uncle Sam gets to keep the cash.

The IRS estimates that the median refund -- meaning half of the checks will be larger and half smaller -- is $581 and the money is owed to nonfiling taxpayers in every state and the District of Columbia, as well as to residents of U.S. territories and military filers who didn't file returns that year.

5 states with most unclaimed refunds

California is home to the most taxpayers, almost 155,000, who didn't send in returns years ago, accounting for more than $144 million of the total unclaimed pot. But New Hampshire residents will likely collect larger checks. The Granite State's median unclaimed refund is $667.

Technically, the biggest refunds, with a median unclaimed check of $800, are owed to 5,500 nonfiling members of the armed forces who are stationed in various locations.

For a full list of all 50 states and their median unclaimed returns, see "Unclaimed tax refunds by state."

Not an uncommon oversight
Despite the staggering amount, the IRS says that unclaimed refund money is not that unusual.

Each year, some people don't file a return because they don't owe taxes. But without the documentation, these folks won't get any refunds they're due. The IRS doesn't send refunds until it gets a Form 1040 (or 1040A or 1040EZ) that details just how big the government's check should be.

Other taxpayers had income tax withheld but didn't have to file a return because they earned too little. But the only way they can get the withheld money back is to file a return.

Then there are individuals who were eligible for but didn't take the earned income tax credit. This is a tax break for workers who don't earn much. It can actually rebate cash to qualified filers -- even those who owe nothing. In 2005, individuals qualified for the earned income credit if they made less than $11,750 and were childless; earned less than $31,030 and had one child; or earned no more than $35,263 and had two or more kids at home.

No penalty, but no payoff either
There is no penalty for late-filing in cases where the taxpayer is due a refund. However, if you subsequently ran up an IRS bill that you didn't pay or you owe another debt of concern to the government, such as child support or a student loan, you may not get all of your 2005 refund. In these cases, once the nonfiler finally gets a return to the IRS, the tax cash will be applied to outstanding charges before any remainder is refunded.

If you think some of the refund stash is yours, you can download a 2005 Form 1040 from the IRS Web site. If you need an old 1040A or 1040EZ instead, you can find them at the agency's index of past-year forms.

Be sure to check out the 2006 and 2007 documents, too, if you didn't file a return for those years either. The IRS won't send you your 2005 cash unless you filed in subsequent years.

And you'd better do it soon. After April 15, your refund money stays in Uncle Sam's pocket.

Freelance writer Kay Bell writes Bankrate's tax stories from her Austin, Texas, home. She also writes two tax blogs, Eye on the IRS for Bankrate and Don't Mess With Taxes, and is the author of the book "The Truth About Paying Fewer Taxes."