Do we know yet what the filing status was?  There are some important needles in this haystack.

I think somewhere you said there are two kids, but one is not claimed as a dependent.  Is that one over 17 by any chance?  Are you sure that the SSN's for the two kids are not transposed?  That could go on for years until it made a difference to IRS.

Is there another parent who might be claiming the child also?