S.2851: A bill to amend the Internal Revenue Code of 1986 to modify the penalty on the understatement of taxpayer's liability by tax return preparers.

Amends the Internal Revenue Code to modify the standards for imposing penalties on tax return preparers for understatements of tax to require: (1) substantial authority for a position with respect to an item on a tax return if such position was not disclosed with the return; and (2) a reasonable basis for a position which was disclosed with the return.

Requires tax return preparers to have a reasonable belief that a position with respect to a tax shelter or a reportable transaction (a transaction having a potential for tax avoidance or evasion) will more likely than not be sustained on its merits.