5 C.F.R. § 839.804 – If my qualifying retirement coverage error occurred while I was a reemployed annuitant, and I am later corrected retroactively to a different retirement plan, will I have to pay any additional amount for retirement deductions?

(a) If you (as a reemployed annuitant) were erroneously in CSRS and had retirement deductions withheld from pay, and later are corrected to CSRS Offset or FERS coverage, the amount erroneously withheld under CSRS will be used to pay the retroactive CSRS Offset or FERS retirement deductions and Social Security taxes.
(b) If you (as a reemployed annuitant) were erroneously placed in CSRS and elected not to have retirement deductions withheld from pay, and later are corrected to CSRS Offset or FERS, your share of retroactive Social Security taxes will be treated as an overpayment of salary. If you are corrected to CSRS Offset, you may elect to have retirement deductions withheld from future salary as a reemployed annuitant and may also make a deposit to cover the retirement deductions for past service as a reemployed annuitant in accordance with § 837.503(c) of this chapter. If you are corrected to FERS, your retirement deductions under FERS will be treated as an overpayment of salary.
(c) If you (as a reemployed annuitant) were erroneously in CSRS Offset and had retirement deductions withheld from pay, and later are corrected to CSRS or FERS coverage, the amount erroneously withheld under CSRS Offset will be used to pay the retroactive CSRS or FERS retirement deductions. The employer is responsible for paying to the Fund any additional retirement deductions.
(d) If you (as a reemployed annuitant) were erroneously placed in CSRS Offset and elected not to have retirement deductions withheld from pay, and later are corrected to CSRS, you may elect to have retirement deductions withheld from future salary as a reemployed annuitant and may also make a deposit to cover the retirement deductions for past service as a reemployed annuitant in accordance with § 837.503(c) of this chapter. Your retirement deductions under CSRS will be treated as an overpayment of salary.
(e) If you (as a reemployed annuitant) were erroneously placed in CSRS Offset and elected not to have retirement deductions withheld from pay, and later are corrected to FERS, your retirement deductions under FERS will be treated as an overpayment of salary.
(f) A reemployed annuitant erroneously placed in FERS and later corrected to CSRS or CSRS Offset is considered to have elected retirement deductions as a reemployed annuitant under the corrected coverage. The employer is responsible for paying to the Fund any additional retirement deductions under the corrected retirement coverage.
(g) If you have a salary overpayment, your employer will inform you of your rights regarding the overpayment.

(h) These rules are summarized in the following chart:

Wrong coverage is:	And retirement deductions were	And you arecorrected to	Then
(1) CSRS	Taken	CSRS Offset or FERS	• The erroneous CSRS deductions are used to pay the retroactive CSRS Offset or FERS deductions and Social Security taxes.• Retirement deductions will continue to be withheld from salary.• Social Security taxes must be withheld from salary.
(2) CSRS	Not taken	CSRS Offset	• Retroactive Social Security taxes are treated as an overpayment of salary.• You may elect to have retirement deductions withheld from future salary.• Social Security taxes must be withheld from salary.• You may pay a deposit to OPM for past retirement deductions.
(3) CSRS	Not taken	FERS	• Retroactive Social Security taxes are treated as an overpayment of salary.• Retirement deductions and Social Security taxes must be withheld from salary.• Your retirement deductions for past service under FERS are treated as an overpayment of salary.
(4) CSRS Offset	Taken	CSRS or FERS	• The erroneous CSRS Offset deductions are used to pay retroactive CSRS or FERS retirement deductions.• Retirement deductions will continue to be withheld from salary.• Social Security taxes must be withheld from salary if correct coverage is FERS.• Employer must pay any additional amount of retirement deductions.
(5) CSRS Offset	Not taken	CSRS	• You may elect to have retirement deductions withheld from future salary.• You may pay a deposit to OPM for past retirement deductions.
(6) CSRS Offset	Not taken	FERS	• Your retirement deductions for past service under FERS will be treated as an overpayment of salary.
(7) FERS	Taken	CSRS or CSRS Offset	• You are considered to have elected retirement deductions as a reemployed annuitant under the corrected coverage.• Employer must pay any additional retirement deductions due for past service.