This actually happened to me. I owed $100.00 in 1991 and sent my return along with a check for $100.00. I received a letter from the IRS stating I had made a mistake and they were returning my check along with a refund check for $900.00. I checked my return, a 1040EZ, and found that the IRS was wrong and I was right so I returned the refund check and my original check by certified mail to them with a letter explaining their error. To make this short, the IRS and I sent the check back and forth 3 times with the IRS telling me they were right. The forth time I received the refund, I gave up and put it in a savings account and didn't touch it. I also kept all the correspondence and certified mail receipts and cards and the IRS' letters telling me they were right and I was wrong. A year later, I received a certified letter from them saying they found an "error" and I now owed them over $3500.00 with penalties and interest. When I went to the IRS office to discuss it, even with all the correspondence where I tried to get them to take it back they would only reduce the amount to around $1500.00 and basically said I should have been more persistent. Their "error" cost me around $600.00! They got you no matter what!