5 C.F.R. § 839.401 What can I elect if I was put in CSRS or CSRS Offset by mistake?
If you were placed in CSRS or CSRS Offset due to a qualifying retirement coverage error and you should have been in FERS, you may elect CSRS Offset or FERS. If you were placed in CSRS or CSRS Offset due to a qualifying retirement coverage error and you should have been in Social Security-Only, you may elect CSRS Offset or Social Security-Only. This is summarized in the following chart:
