I missed this credit and I want to share my lesson so others can learn.

TSP is a qualifying retirement plan under Retirement Savings Contributions Credit (formerly called the Saver’s Tax Credit).
TSP website

If you are under this bracket:
Single, Adjusted Gross Income:
0-15500: 50%
15501-17000: 20%
17001-26000: 10%

Head of Household, Adjusted Gross Income:
0-23250: 50%
23251-25500: 20%
25501-39000: 10%

Joint File, Adjusted Gross Income:
0-31000: 50%
31001-34000: 20%
34001-52000: 10%

So, if you're single and your AGI is less than $15500 (E-1 maybe?), you get 50 cents in tax credit for every dollar you put in TSP (max $2000).

My goodness, I filed 1040EZ for 2002, 2003, and 2004 with contributions to TSP every year without claiming this credit.

You may file amended returns for the yaars you missed claiming the credit. Whatever you overpaid in tax will be refunded to you PLUS interest!!-

This is true only for those returns which are amended within three years of the original due date of the returns.

'03 was due in '04. You may no longer get any refunds for that year. '04,'05 and '06 are stilllgoo to go!!=

I am gonna to amend my 2004. My 2005 AGI was beyond the single AGI bracket.

Great. My AGI was above the $25,000 limit (2004). I could have contributed to Traditional IRA and bring it down to $25k.

Should have learned about personal finance one year early and I could cut my tax more than $200 in 2004.

If you are married and the sole wage erarner in your household and you are E-5 and below and you are receiving money back at the end of the year, then you need to re-adjust your w-4. Especially if you have children. Most get money back and don't put anything in due to Earned Income Credit. Keep your money and put it to where it needs to go. Your savings or paying down consumer debt so that you may start preparing for retirement. What good is it to put money into a retirement fund earning seven percent when your consumer bills are at 19%. Get the debt gone first. then double up on retirement.