N.J. Admin. Code § 17:6-16.9
If any contribution (or any portion of a contribution) is made by the employer by a good faith mistake of fact, upon receipt in good order of a proper request by the Plan Administrator, the Trustee or the DSP shall return the amount of the mistaken contribution(s), except as limited below, to the employer. A return of a mistaken contribution will not be made if the return will not be made within one year from the date of the mistaken payment of the contribution, or if the participant has had a severance from employment and received the distribution of his or her account. Upon any return of a mistaken contribution, earnings attributable to the mistaken contributions will not be returned and losses attributable to the mistaken contribution shall reduce the amount to be returned.
N.J. Admin. Code § 17:6-16.9