5 CFR § 839.901 - When will my employer begin withholding Social Security taxes if I was erroneously in CSRS during my qualifying retirement coverage error and my corrected coverage will now require me to pay Social Security taxes?

(a) If you are in CSRS by mistake and belong in CSRS Offset, FERS, or Social Security-Only, your employer must begin withholding Social Security taxes by changing your retirement coverage to CSRS Offset. Your employer must begin this withholding as soon as possible after the error is discovered.

(b) Your employer will correct your retirement coverage back to the date the error first occurred once you are notified of the error and have an opportunity to make any elections that you are eligible to make.