A Supreme Court case that struck down the Line Item Veto Act because it gave the executive the unilateral authority to amend a law without having to go through the legislative process. The Line Item Veto Act, intended by Congress to limit government spending, allowed the President to veto a single appropriation or tax benefit within a large appropriation or tax bill without having to veto the entire bill. The Supreme Court ruled this Act to be unconstitutional because it violated the Presentment Clause of the Constitution (which delineated a process by which a piece of legislation passed through both houses of Congress in its entirety before being signed or vetoed in its entirety by the President). By giving the President the discretion to accept or reject parts of the bill, the Act would have allowed the President to act contrary to Congressional purpose by amending the laws that were presented to him.
See Clinton v. City of New York, 524 U.S. 417 (1998) at http://www.law.cornell.edu/supct/html/97-1374.ZO.html