I filed my 2011 taxes on-time and owed about $1000, which I paid with my return. In June I received a check from the US treasury department for $140 for an 'overtax refund'. Besides that one line on the check there was no explanation. I looked over my return and could not find any errors. I was expecting at least a letter with an explanation.

I'm a bit stumped as to 1. Why I would receive a refund when my return showed an amount due and 2. What mistake I made (if any) to cause the refund. My other idea is that this might be for a prior tax year, but I have not filed any amended returns for previous year.

I did file an amended state return 2 years ago but I don't know how that would affect my federal return.