I received a form in the mail from our state comptroller's office notifying me of a calculation error on our tax form. Now we're getting back $1400 less from our state taxes than we thought. Oops.
I once got a form like that back from the federal government. I think the mistake was in their favor so I actually got back more than I had calculated. Ever since then, I've been a little less leery of making mistakes because I know that their computers will find them and fix them for me without sending armed guards to my home or place of business.