Why do I have an estimated tax penalty, when I've never paid or had to file estimated taxes?

[GBillMcI]

It sounds like you might've gotten caught by the mathematics of your income this year.  You must meet one of 3 hurdles to avoid a tax payment penalty.  You must:

1 - Owe less than $1,000 when filing your current year tax return;

2 - Have already paid or had withheld at least 90% of your total current year taxes due;

OR

3 - Have already paid or had withheld at least 100% of your total taxes due for last year.

If you failed all 3 tests, then you are subject to a tax penalty.  

[MaryK4]

The United States income tax system is a pay-as-you-go tax system, which means that you must pay income tax as you earn or receive your income during the year. You can do this either through withholding or by making estimated tax payments. If you didn't pay enough tax throughout the year, either through withholding or by making estimated tax payments, you may have to pay a penalty for underpayment of estimated tax. Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and refundable credits, or if they paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller.