If you filed a tax return, the IRS only has three years from the due date (or two years from the date you actually filed, whichever is later) to challenge you. Assuming that you filed your 2012 return by the due date, the time to challenge you has passed.
When the IRS sends you a letter saying that you owe more money, they will tell you exactly why. They found another W-2. They found a 1099-C. They will go through the calculations to show how they arrived at the balance due.
The letter goes on to give you places to sign saying that you agree, you partially agree, or you disagree.
If you filed your tax return by the due date (April 15, 2013), then this letter may be fraudulent and should be reported to the IRS. If your tax return was rejected, then officially, you haven't filed, and there is no limit to how long the IRS has to pursue collection.
Be very careful how you respond. There should be a phone number for you to call, so you can get your questions answered. Your Social Security Number will be on the letter. If you have any doubts about whether this letter is genuine, please call the number on the letter. The agent who responds will give you their name and ID number. Write them down. Don't share any information unless you are convinced that this letter is genuine.
