Did Turbotax calculate my federal taxes owed wrongly? I received a check out of nowhere.

[mren]

Out of nowhere came a check from the treasury with a refund. I must have overpaid my taxes. How can that be? Did turbotax make an error? There is no additional paperwork, just a check. How would I ever find out what went wrong in my favor?

Thanks!

[VolvoGirl]

They will mail you a letter in a couple weeks.  Some things to check.  They may have paid you interest.  They may have reduced a penalty on 1040 line 38.  You may have entered some withholding wrong or missed entering an estimated payment or extension payment.