Back Taxes Owed
You filed your returns but did not have the money to pay what was owed. You decided - "Oh well - I will catch up next tax season." Before you know it, several years later you are holding a notice from the IRS, stating that you owe three or four times the original amount...
It's amazing how fast tax penalties and interest add up. You could write a big fat check and pay the full amount, including interest and penalties. Or might decide to keep ignoring the IRS while penalties and interest keep piling up.