5 C.F.R. § 839.401

What can I elect if I was put in CSRS or CSRS Offset by mistake?

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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:

You are in:And you belong in:You may elect:
CSRSFERSCSRS Offset or FERS.
CSRS OffsetFERS
CSRSSocial Security-OnlyCSRS Offset or Social Security-Only.
CSRS OffsetSocial Security-Only