On July 21, 2007, Dick Cheney officially became the president of the United States.
Well, for two hours and five minutes. Due to a colonoscopy that took place from 7:16 a.m. to 9:21 a.m., George W. Bush was, in the words of section three of the 25th amendment to the U.S. Constitution, "unable to discharge the powers and duties of his office," and with a letter in accordance with the 1967 amendment, passed his presidential duties to Cheney as "Acting" President.
The list of Acting Presidents is a short one, with Cheney occupying two of the three spots. Yet while Bush's 2002 and 2007 transfers of power were fairly straightforward, Ronald Reagan's 1985 letter handing over the reins to George H.W. Bush is still contested — because of his expressed reluctance to set a precedent for the amendment's use, some argue that Reagan's transfer of power was invalid.
Indeed, history shows that presidents are generally unenthusiastic about relinquishing their posts, even briefly. Despite being seriously injured after a 1981 assassination attempt, Reagan did not invoke the 25th amendment. George H.W. Bush twice made plans to transfer power to Vice President Dan Quayle during periods of illness and irregular heartbeat, but no official handover of power was ever made.
In fact, if you go strictly by the books, it would seem that the colon is the biggest threat to a president retaining his duties. Bush Sr.'s hours of power came when Reagan underwent colon cancer surgery, and both of W.'s handovers to Cheney were due to colonoscopies.