I just figured out that I made a mistake in my 2006 income taxes, and didn't file for the 'Additional Child Tax Credit' form 8812.... which cost me $844!
Can you amend 2006 tax return and claim an ommitted tax credit?
You can absolutely file an amended tax return and claim the tax credit that was omitted. You must follow the proper procedure and carefully prepare the amended return without mistakes or else the return may not get processed at all.
If in doubt, definitely seek the advice of a professional tax preparer to ensure that this amended tax return is prepared correctly and accurately for a prompt refund.