31 CFR 10.21 : Money And Finance: Treasury (§ 0.101 To Parts 1061-1099) — (§ 0.101 To Parts 151-199) — Office Of The Secretary Of The Treasury (§ 0.101 To § 50.95) — (§ 0.101 To § 50.95) — Practice Before The Internal Revenue Service (§ 10.0 To § 10.93) — Duties And Restrictions Relating To Practice Before The Internal Revenue Service (§ 10.20 To § 10.38) — Knowledge of client's omission.

A practitioner who, having been retained by a client with respect to a matter administered by the Internal Revenue Service, knows that the client has not complied with the revenue laws of the United States or has made an error in or omission from any return, document, affidavit, or other paper which the client submitted or executed under the revenue laws of the United States, must advise the client promptly of the fact of such noncompliance, error, or omission. The practitioner must advise the client of the consequences as provided under the Code and regulations of such noncompliance, error, or omission.