IRS: What happens if you're late on your estimates?

Answer 1

You may be subject to penalties for underpayment of estimated tax. When filing your tax return, you should be able to enter in the amount and the date of the tax payment. The software will calculate any penalties/interest due. You will need to attach Form 2210 to your tax return, which the software should generate.

http://www.irs.gov/pub/irs-pdf/f2210.pdf

Answer 2

Estimated tax payments have a specific date by which they must be paid for a specific period. If your income during that specific period would result in more tax than you have provided they will subject you to a penalty.