Twenty-fifth Amendment to the United States Constitution
On March 30, 1981, a man named John Hinckley tried to kill President Ronald Reagan.  Reagan was shot and needed immediate surgery, so he could not invoke Section 3 to give power to his Vice President.  His Vice President, George H.W. Bush, did not invoke Section 4, because he was on an airplane returning from Texas.  Reagan was out of surgery by the time Bush got to Washington, D.C.pp. 195–6, 253-5