United States v. Nixon 1974

The president of the United States taped many conversation in the White House's Oval Office, so the special prosecutor demanded the tape, but president Nixon refused to surrender the tapes because he thinks he had executive privilege. However, the Court agreed unanimously that the President had to turn over the tapes.
And I think the Court made a right decision, because even the president has to live under the same law as regular citizen. No matter who it is, we cannot give anybody advantage.
For more information, we could buy that book, and learn more about this case.
 President thought the executive privilege was secure from a subpoena because it was necessary to protect the confidentiality of high-level communications, and the principle of separation of powers protects the president through the independence of Executive Branch, but the Court found this argument insufficient.