You may wish to see a lawyer because the IRS 25% safe harbor applies only to missed payments extending more than three months. My reading of the IRS rules, including the Appendix, suggests that, in the case of the shorter period you describe, the erroneous payments should be corrected in whole, e.g., by deduction from your next pay on 12/27. You can also have a lawyer discuss whether there is a fiduciary breach by the employer under ERISA in failing to follow plan documents, including employee elections, even though compliance with an IRS safe harbor protects the plan from disqualification.