If you make a mistake on your taxes and need to amend a tax return, you will fill out IRS Form 1040X.  If you make a mistake on the amended tax return, you can even amend the amended return.

To amend a tax return, you have three years from the date the original tax return was filed (or its due date).  For example, assume it is the year 2050 and the tax rules are the same as today.  If John Doe filed his taxes for 2049 on April 2, 2050 (13 days before the due date), then the last date that John could amend his return would be April 15,  2053.  This is exactly three years after the initial due date.  This means that 2006 tax information would be available to April 15, 2010 then removed.  2007 information would be removed after April 15, 2011, and so on.