I have a merge statement that returns 111 return code, but appears to have worked.

Code:
merge 1:m _taxsimid state year using `original'

Result                           # of obs.
    -----------------------------------------
    not matched                             0
    matched                                 1  (_merge==3)
    -----------------------------------------
Merge failed 111

                 _merge |      Freq.     Percent        Cum.
------------------------+-----------------------------------
            matched (3) |          1      100.00      100.00
------------------------+-----------------------------------
                  Total |          1      100.00
Note that all (one) of the records matched, and I can list the record to confirm that the merge was done correctly. So what could be the problem causing the "Merge failed 111" error message?