5 C.F.R. § 839.411

What can I elect if I was put in FERS by mistake?

If you were placed in FERS due to a qualifying retirement coverage error and you should have been in CSRS, you may elect FERS or CSRS. If you were placed in FERS due to a qualifying retirement coverage error and you should have been in CSRS Offset, you may elect FERS or CSRS Offset. If you were placed in FERS due to a qualifying retirement coverage error and you should have been in Social Security-Only, you may elect FERS or Social Security-Only. This is summarized in the following chart:

You are in: | And you belong in: | You may elect:; FERS | CSRS | FERS or CSRS.; FERS | CSRS Offset | FERS or CSRS Offset.; FERS | Social Security-Only | FERS or Social Security-Only.