Does this sound familiar? A few years back your yearly earnings were pretty low so you figured you wouldn't owe any income tax. Thus, when April 15 rolled around the following year you didn't bother filing a tax return, knowing you wouldn't be penalized. Big mistake.
Although it's tempting to let such mistakes slide, chances are the IRS will discover the error eventually, and when they do you could be liable for interest and penalties going back to the due date of the original tax return.